]> git.xonotic.org Git - xonotic/gmqcc.git/blobdiff - fold.c
Constant fold intrinsics if their arguments are constant. TODO: reference count intri...
[xonotic/gmqcc.git] / fold.c
diff --git a/fold.c b/fold.c
index 662601bc0ab61a8c91478f6d38cc46e5a8c8c4a9..360b4f0f86e0cbd5866f2e6dc6fb5f5a0b8533e3 100644 (file)
--- a/fold.c
+++ b/fold.c
@@ -34,7 +34,7 @@
  * stage constant folding, where, witht he help of the AST, operator
  * usages can be constant folded. Then there is the constant folding
  * in the IR for things like eliding if statements, can occur.
- * 
+ *
  * This file is thus, split into two parts.
  */
 
 #define isvector(X)     (((ast_expression*)(X))->vtype == TYPE_VECTOR)
 #define isstring(X)     (((ast_expression*)(X))->vtype == TYPE_STRING)
 #define isfloats(X,Y)   (isfloat  (X) && isfloat (Y))
-#define isvectors(X,Y)  (isvector (X) && isvector(Y))
 
 /*
  * Implementation of basic vector math for vec3_t, for trivial constant
  * folding.
- * 
+ *
  * TODO: gcc/clang hinting for autovectorization
  */
 static GMQCC_INLINE vec3_t vec3_add(vec3_t a, vec3_t b) {
@@ -74,19 +73,59 @@ static GMQCC_INLINE vec3_t vec3_neg(vec3_t a) {
     return out;
 }
 
+static GMQCC_INLINE vec3_t vec3_or(vec3_t a, vec3_t b) {
+    vec3_t out;
+    out.x = (qcfloat_t)(((qcint_t)a.x) | ((qcint_t)b.x));
+    out.y = (qcfloat_t)(((qcint_t)a.y) | ((qcint_t)b.y));
+    out.z = (qcfloat_t)(((qcint_t)a.z) | ((qcint_t)b.z));
+    return out;
+}
+
+static GMQCC_INLINE vec3_t vec3_orvf(vec3_t a, qcfloat_t b) {
+    vec3_t out;
+    out.x = (qcfloat_t)(((qcint_t)a.x) | ((qcint_t)b));
+    out.y = (qcfloat_t)(((qcint_t)a.y) | ((qcint_t)b));
+    out.z = (qcfloat_t)(((qcint_t)a.z) | ((qcint_t)b));
+    return out;
+}
+
+static GMQCC_INLINE vec3_t vec3_and(vec3_t a, vec3_t b) {
+    vec3_t out;
+    out.x = (qcfloat_t)(((qcint_t)a.x) & ((qcint_t)b.x));
+    out.y = (qcfloat_t)(((qcint_t)a.y) & ((qcint_t)b.y));
+    out.z = (qcfloat_t)(((qcint_t)a.z) & ((qcint_t)b.z));
+    return out;
+}
+
+static GMQCC_INLINE vec3_t vec3_andvf(vec3_t a, qcfloat_t b) {
+    vec3_t out;
+    out.x = (qcfloat_t)(((qcint_t)a.x) & ((qcint_t)b));
+    out.y = (qcfloat_t)(((qcint_t)a.y) & ((qcint_t)b));
+    out.z = (qcfloat_t)(((qcint_t)a.z) & ((qcint_t)b));
+    return out;
+}
+
 static GMQCC_INLINE vec3_t vec3_xor(vec3_t a, vec3_t b) {
     vec3_t out;
-    out.x = (qcfloat_t)((qcint_t)a.x ^ (qcint_t)b.x);
-    out.y = (qcfloat_t)((qcint_t)a.y ^ (qcint_t)b.y);
-    out.z = (qcfloat_t)((qcint_t)a.z ^ (qcint_t)b.z);
+    out.x = (qcfloat_t)(((qcint_t)a.x) ^ ((qcint_t)b.x));
+    out.y = (qcfloat_t)(((qcint_t)a.y) ^ ((qcint_t)b.y));
+    out.z = (qcfloat_t)(((qcint_t)a.z) ^ ((qcint_t)b.z));
     return out;
 }
 
 static GMQCC_INLINE vec3_t vec3_xorvf(vec3_t a, qcfloat_t b) {
     vec3_t out;
-    out.x = (qcfloat_t)((qcint_t)a.x ^ (qcint_t)b);
-    out.y = (qcfloat_t)((qcint_t)a.y ^ (qcint_t)b);
-    out.z = (qcfloat_t)((qcint_t)a.z ^ (qcint_t)b);
+    out.x = (qcfloat_t)(((qcint_t)a.x) ^ ((qcint_t)b));
+    out.y = (qcfloat_t)(((qcint_t)a.y) ^ ((qcint_t)b));
+    out.z = (qcfloat_t)(((qcint_t)a.z) ^ ((qcint_t)b));
+    return out;
+}
+
+static GMQCC_INLINE vec3_t vec3_not(vec3_t a) {
+    vec3_t out;
+    out.x = (qcfloat_t)(~((qcint_t)a.x));
+    out.y = (qcfloat_t)(~((qcint_t)a.y));
+    out.z = (qcfloat_t)(~((qcint_t)a.z));
     return out;
 }
 
@@ -124,14 +163,6 @@ static GMQCC_INLINE bool vec3_pbool(vec3_t a) {
     return (a.x && a.y && a.z);
 }
 
-static GMQCC_INLINE bool fold_can_1(const ast_value *val) {
-    return  (ast_istype((ast_expression*)val, ast_value) && val->hasvalue && val->cvq == CV_CONST && ((ast_expression*)val)->vtype != TYPE_FUNCTION);
-}
-
-static GMQCC_INLINE bool fold_can_2(const ast_value *v1, const ast_value *v2) {
-    return fold_can_1(v1) && fold_can_1(v2);
-}
-
 static lex_ctx_t fold_ctx(fold_t *fold) {
     lex_ctx_t ctx;
     if (fold->parser->lex)
@@ -147,10 +178,10 @@ static GMQCC_INLINE bool fold_immediate_true(fold_t *fold, ast_value *v) {
             return !!v->constval.vfloat;
         case TYPE_INTEGER:
             return !!v->constval.vint;
-        case TYPE_VECTOR: 
+        case TYPE_VECTOR:
             if (OPTS_FLAG(CORRECT_LOGIC))
                 return vec3_pbool(v->constval.vvec);
-            return !!v->constval.vvec.x;
+            return !!(v->constval.vvec.x);
         case TYPE_STRING:
             if (!v->constval.vstring)
                 return false;
@@ -164,6 +195,13 @@ static GMQCC_INLINE bool fold_immediate_true(fold_t *fold, ast_value *v) {
     return !!v->constval.vfunc;
 }
 
+/* Handy macros to determine if an ast_value can be constant folded. */
+#define fold_can_1(X)  \
+    (ast_istype(((ast_expression*)(X)), ast_value) && (X)->hasvalue && ((X)->cvq == CV_CONST) && \
+                ((ast_expression*)(X))->vtype != TYPE_FUNCTION)
+
+#define fold_can_2(X, Y) (fold_can_1(X) && fold_can_1(Y))
+
 #define fold_immvalue_float(E)  ((E)->constval.vfloat)
 #define fold_immvalue_vector(E) ((E)->constval.vvec)
 #define fold_immvalue_string(E) ((E)->constval.vstring)
@@ -186,6 +224,7 @@ fold_t *fold_init(parser_t *parser) {
     (void)fold_constgen_float (fold, -1.0f);
 
     (void)fold_constgen_vector(fold, vec3_create(0.0f, 0.0f, 0.0f));
+    (void)fold_constgen_vector(fold, vec3_create(-1.0f, -1.0f, -1.0f));
 
     return fold;
 }
@@ -297,19 +336,19 @@ static GMQCC_INLINE ast_expression *fold_op_mul_vec(fold_t *fold, vec3_t vec, as
     /*
      * vector-component constant folding works by matching the component sets
      * to eliminate expensive operations on whole-vectors (3 components at runtime).
-     * to achive this effect in a clean manner this function generalizes the 
+     * to achive this effect in a clean manner this function generalizes the
      * values through the use of a set paramater, which is used as an indexing method
      * for creating the elided ast binary expression.
      *
      * Consider 'n 0 0' where y, and z need to be tested for 0, and x is
-     * used as the value in a binary operation generating an INSTR_MUL instruction
+     * used as the value in a binary operation generating an INSTR_MUL instruction,
      * to acomplish the indexing of the correct component value we use set[0], set[1], set[2]
      * as x, y, z, where the values of those operations return 'x', 'y', 'z'. Because
      * of how ASCII works we can easily deliniate:
      * vec.z is the same as set[2]-'x' for when set[2] is 'z', 'z'-'x' results in a
      * literal value of 2, using this 2, we know that taking the address of vec->x (float)
      * and indxing it with this literal will yeild the immediate address of that component
-     * 
+     *
      * Of course more work needs to be done to generate the correct index for the ast_member_new
      * call, which is no problem: set[0]-'x' suffices that job.
      */
@@ -323,7 +362,7 @@ static GMQCC_INLINE ast_expression *fold_op_mul_vec(fold_t *fold, vec3_t vec, as
         out                        = (ast_expression*)ast_member_new(fold_ctx(fold), (ast_expression*)sel, set[0]-'x', NULL);
         out->node.keep             = false;
         ((ast_member*)out)->rvalue = true;
-        if (!x != -1)
+        if (x != -1)
             return (ast_expression*)ast_binary_new(fold_ctx(fold), INSTR_MUL_F, fold_constgen_float(fold, x), out);
     }
     return NULL;
@@ -383,13 +422,17 @@ static GMQCC_INLINE ast_expression *fold_op_sub(fold_t *fold, ast_value *a, ast_
 
 static GMQCC_INLINE ast_expression *fold_op_mul(fold_t *fold, ast_value *a, ast_value *b) {
     if (isfloat(a)) {
-        if (isfloat(b) && fold_can_2(a, b))
-            return fold_constgen_vector(fold, vec3_mulvf(fold_immvalue_vector(b), fold_immvalue_float(a)));
-        else if (fold_can_2(a, b))
-            return fold_constgen_float(fold, fold_immvalue_float(a) * fold_immvalue_float(b));
+        if (isvector(b)) {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_mulvf(fold_immvalue_vector(b), fold_immvalue_float(a)));
+        } else {
+            if (fold_can_2(a, b))
+                return fold_constgen_float(fold, fold_immvalue_float(a) * fold_immvalue_float(b));
+        }
     } else if (isvector(a)) {
-        if (isfloat(b) && fold_can_2(a, b)) {
-            return fold_constgen_vector(fold, vec3_mulvf(fold_immvalue_vector(a), fold_immvalue_float(b)));
+        if (isfloat(b)) {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_mulvf(fold_immvalue_vector(a), fold_immvalue_float(b)));
         } else {
             if (fold_can_2(a, b)) {
                 return fold_constgen_float(fold, vec3_mulvv(fold_immvalue_vector(a), fold_immvalue_vector(b)));
@@ -416,8 +459,21 @@ static GMQCC_INLINE ast_expression *fold_op_div(fold_t *fold, ast_value *a, ast_
     } else if (isvector(a)) {
         if (fold_can_2(a, b))
             return fold_constgen_vector(fold, vec3_mulvf(fold_immvalue_vector(a), 1.0f / fold_immvalue_float(b)));
-        else if (fold_can_1(b))
-            return fold_constgen_float (fold, 1.0f / fold_immvalue_float(b));
+        else {
+            return (ast_expression*)ast_binary_new(
+                fold_ctx(fold),
+                INSTR_MUL_VF,
+                (ast_expression*)a,
+                (fold_can_1(b))
+                    ? (ast_expression*)fold_constgen_float(fold, 1.0f / fold_immvalue_float(b))
+                    : (ast_expression*)ast_binary_new(
+                                            fold_ctx(fold),
+                                            INSTR_DIV_F,
+                                            (ast_expression*)fold->imm_float[1],
+                                            (ast_expression*)b
+                    )
+            );
+        }
     }
     return NULL;
 }
@@ -429,14 +485,34 @@ static GMQCC_INLINE ast_expression *fold_op_mod(fold_t *fold, ast_value *a, ast_
 }
 
 static GMQCC_INLINE ast_expression *fold_op_bor(fold_t *fold, ast_value *a, ast_value *b) {
-    if (fold_can_2(a, b))
-        return fold_constgen_float(fold, (qcfloat_t)(((qcint_t)fold_immvalue_float(a)) | ((qcint_t)fold_immvalue_float(b))));
+    if (isfloat(a)) {
+        if (fold_can_2(a, b))
+            return fold_constgen_float(fold, (qcfloat_t)(((qcint_t)fold_immvalue_float(a)) | ((qcint_t)fold_immvalue_float(b))));
+    } else {
+        if (isvector(b)) {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_or(fold_immvalue_vector(a), fold_immvalue_vector(b)));
+        } else {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_orvf(fold_immvalue_vector(a), fold_immvalue_float(b)));
+        }
+    }
     return NULL;
 }
 
 static GMQCC_INLINE ast_expression *fold_op_band(fold_t *fold, ast_value *a, ast_value *b) {
-    if (fold_can_2(a, b))
-        return fold_constgen_float(fold, (qcfloat_t)(((qcint_t)fold_immvalue_float(a)) & ((qcint_t)fold_immvalue_float(b))));
+    if (isfloat(a)) {
+        if (fold_can_2(a, b))
+            return fold_constgen_float(fold, (qcfloat_t)(((qcint_t)fold_immvalue_float(a)) & ((qcint_t)fold_immvalue_float(b))));
+    } else {
+        if (isvector(b)) {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_and(fold_immvalue_vector(a), fold_immvalue_vector(b)));
+        } else {
+            if (fold_can_2(a, b))
+                return fold_constgen_vector(fold, vec3_andvf(fold_immvalue_vector(a), fold_immvalue_float(b)));
+        }
+    }
     return NULL;
 }
 
@@ -468,18 +544,18 @@ static GMQCC_INLINE ast_expression *fold_op_rshift(fold_t *fold, ast_value *a, a
     return NULL;
 }
 
-static GMQCC_INLINE ast_expression *fold_op_andor(fold_t *fold, ast_value *a, ast_value *b, float or) {
+static GMQCC_INLINE ast_expression *fold_op_andor(fold_t *fold, ast_value *a, ast_value *b, float expr) {
     if (fold_can_2(a, b)) {
         if (OPTS_FLAG(PERL_LOGIC)) {
             if (fold_immediate_true(fold, a))
                 return (ast_expression*)b;
         } else {
             return fold_constgen_float (
-                fold, 
-                ((or) ? (fold_immediate_true(fold, a) || fold_immediate_true(fold, b))
-                      : (fold_immediate_true(fold, a) && fold_immediate_true(fold, b)))
-                            ? 1.0f
-                            : 0.0f
+                fold,
+                ((expr) ? (fold_immediate_true(fold, a) || fold_immediate_true(fold, b))
+                        : (fold_immediate_true(fold, a) && fold_immediate_true(fold, b)))
+                            ? 1
+                            : 0
             );
         }
     }
@@ -522,15 +598,23 @@ static GMQCC_INLINE ast_expression *fold_op_cmp(fold_t *fold, ast_value *a, ast_
 }
 
 static GMQCC_INLINE ast_expression *fold_op_bnot(fold_t *fold, ast_value *a) {
-    if (fold_can_1(a))
-        return fold_constgen_float(fold, ~((qcint_t)fold_immvalue_float(a)));
+    if (isfloat(a)) {
+        if (fold_can_1(a))
+            return fold_constgen_float(fold, ~((qcint_t)fold_immvalue_float(a)));
+    } else {
+        if (isvector(a)) {
+            if (fold_can_1(a))
+                return fold_constgen_vector(fold, vec3_not(fold_immvalue_vector(a)));
+        }
+    }
     return NULL;
 }
 
 ast_expression *fold_op(fold_t *fold, const oper_info *info, ast_expression **opexprs) {
-    ast_value *a = (ast_value*)opexprs[0];
-    ast_value *b = (ast_value*)opexprs[1];
-    ast_value *c = (ast_value*)opexprs[2];
+    ast_value      *a = (ast_value*)opexprs[0];
+    ast_value      *b = (ast_value*)opexprs[1];
+    ast_value      *c = (ast_value*)opexprs[2];
+    ast_expression *e = NULL;
 
     /* can a fold operation be applied to this operator usage? */
     if (!info->folds)
@@ -541,32 +625,172 @@ ast_expression *fold_op(fold_t *fold, const oper_info *info, ast_expression **op
         case 2: if(!b) return NULL;
         case 1:
         if(!a) {
-            compile_error(fold_ctx(fold), "interal error: fold_op no operands to fold\n");
+            compile_error(fold_ctx(fold), "internal error: fold_op no operands to fold\n");
             return NULL;
         }
     }
 
+    /*
+     * we could use a boolean and default case but ironically gcc produces
+     * invalid broken assembly from that operation. clang/tcc get it right,
+     * but interestingly ignore compiling this to a jump-table when I do that,
+     * this happens to be the most efficent method, since you have per-level
+     * granularity on the pointer check happening only for the case you check
+     * it in. Opposed to the default method which would involve a boolean and
+     * pointer check after wards.
+     */
+    #define fold_op_case(ARGS, ARGS_OPID, OP, ARGS_FOLD)    \
+        case opid##ARGS ARGS_OPID:                          \
+            if ((e = fold_op_##OP ARGS_FOLD)) {             \
+                ++opts_optimizationcount[OPTIM_CONST_FOLD]; \
+            }                                               \
+            return e
+
     switch(info->id) {
-        case opid2('-', 'P'):    return fold_op_neg    (fold, a);
-        case opid2('!', 'P'):    return fold_op_not    (fold, a);
-        case opid1('+'):         return fold_op_add    (fold, a, b);
-        case opid1('-'):         return fold_op_sub    (fold, a, b);
-        case opid1('*'):         return fold_op_mul    (fold, a, b);
-        case opid1('/'):         return fold_op_div    (fold, a, b);
-        case opid1('%'):         return fold_op_mod    (fold, a, b);
-        case opid1('|'):         return fold_op_bor    (fold, a, b);
-        case opid1('&'):         return fold_op_band   (fold, a, b);
-        case opid1('^'):         return fold_op_xor    (fold, a, b);
-        case opid2('<','<'):     return fold_op_lshift (fold, a, b);
-        case opid2('>','>'):     return fold_op_rshift (fold, a, b);
-        case opid2('|','|'):     return fold_op_andor  (fold, a, b, true);
-        case opid2('&','&'):     return fold_op_andor  (fold, a, b, false);
-        case opid2('?',':'):     return fold_op_tern   (fold, a, b, c);
-        case opid2('*','*'):     return fold_op_exp    (fold, a, b);
-        case opid3('<','=','>'): return fold_op_lteqgt (fold, a, b);
-        case opid2('!','='):     return fold_op_cmp    (fold, a, b, true);
-        case opid2('=','='):     return fold_op_cmp    (fold, a, b, false);
-        case opid2('~','P'):     return fold_op_bnot   (fold, a);
+        fold_op_case(2, ('-', 'P'),    neg,    (fold, a));
+        fold_op_case(2, ('!', 'P'),    not,    (fold, a));
+        fold_op_case(1, ('+'),         add,    (fold, a, b));
+        fold_op_case(1, ('-'),         sub,    (fold, a, b));
+        fold_op_case(1, ('*'),         mul,    (fold, a, b));
+        fold_op_case(1, ('/'),         div,    (fold, a, b));
+        fold_op_case(1, ('%'),         mod,    (fold, a, b));
+        fold_op_case(1, ('|'),         bor,    (fold, a, b));
+        fold_op_case(1, ('&'),         band,   (fold, a, b));
+        fold_op_case(1, ('^'),         xor,    (fold, a, b));
+        fold_op_case(2, ('<', '<'),    lshift, (fold, a, b));
+        fold_op_case(2, ('>', '>'),    rshift, (fold, a, b));
+        fold_op_case(2, ('|', '|'),    andor,  (fold, a, b, true));
+        fold_op_case(2, ('&', '&'),    andor,  (fold, a, b, false));
+        fold_op_case(2, ('?', ':'),    tern,   (fold, a, b, c));
+        fold_op_case(2, ('*', '*'),    exp,    (fold, a, b));
+        fold_op_case(3, ('<','=','>'), lteqgt, (fold, a, b));
+        fold_op_case(2, ('!', '='),    cmp,    (fold, a, b, true));
+        fold_op_case(2, ('=', '='),    cmp,    (fold, a, b, false));
+        fold_op_case(2, ('~', 'P'),    bnot,   (fold, a));
     }
+    #undef fold_op_case
+    compile_error(fold_ctx(fold), "internal error: attempted to constant-fold for unsupported operator");
     return NULL;
 }
+
+#define expect(X)                                                                                        \
+    do {                                                                                                 \
+        if (vec_size(params) != (X)) {                                                                   \
+            compile_error(                                                                               \
+                fold_ctx(fold),                                                                          \
+                "internal error: attempted to constant-fold with invalid paramaters for intrinsic `%s`", \
+                intrin                                                                                   \
+            );                                                                                           \
+            return NULL;                                                                                 \
+        }                                                                                                \
+    } while (0)
+
+ast_expression *fold_intrin(fold_t *fold, const char *intrin, ast_expression **params) {
+    if (!fold)   return NULL;
+    if (!intrin) return NULL;
+
+    if (!strcmp(intrin, "__builtin_exp")) {
+        expect(1);
+        ++opts_optimizationcount[OPTIM_CONST_FOLD];
+        return fold_constgen_float(fold, exp(fold_immvalue_float((ast_value*)params[0])));
+    }
+
+    if (!strcmp(intrin, "__builtin_mod")) {
+        expect(2);
+        ++opts_optimizationcount[OPTIM_CONST_FOLD];
+        return fold_constgen_float(
+                    fold,
+                    fmodf(
+                        fold_immvalue_float((ast_value*)params[0]),
+                        fold_immvalue_float((ast_value*)params[1])
+                    )
+                );
+    }
+
+    if (!strcmp(intrin, "__builtin_pow")) {
+        expect(2);
+        ++opts_optimizationcount[OPTIM_CONST_FOLD];
+        return fold_constgen_float(
+                    fold,
+                    powf(
+                        fold_immvalue_float((ast_value*)params[0]),
+                        fold_immvalue_float((ast_value*)params[1])
+                    )
+                );
+    }
+
+    if (!strcmp(intrin, "__builtin_isnan")) {
+        expect(1);
+        ++opts_optimizationcount[OPTIM_CONST_FOLD];
+        return fold_constgen_float(fold, isnan(fold_immvalue_float((ast_value*)params[0])) != 0.0f);
+    }
+
+    if (!strcmp(intrin, "__builtin_fabs")) {
+        expect(1);
+        ++opts_optimizationcount[OPTIM_CONST_FOLD];
+        return fold_constgen_float(fold, fabs(fold_immvalue_float((ast_value*)params[0])));
+    }
+
+    return NULL;
+}
+
+/*
+ * These are all the actual constant folding methods that happen in between
+ * the AST/IR stage of the compiler , i.e eliminating branches for const
+ * expressions, which is the only supported thing so far. We undefine the
+ * testing macros here because an ir_value is differant than an ast_value.
+ */
+#undef expect
+#undef isfloat
+#undef isstring
+#undef isvector
+#undef fold_immvalue_float
+#undef fold_immvalue_string
+#undef fold_immvalue_vector
+#undef fold_can_1
+#undef fold_can_2
+
+#define isfloat(X)              ((X)->vtype == TYPE_FLOAT)
+/*#define isstring(X)             ((X)->vtype == TYPE_STRING)*/
+/*#define isvector(X)             ((X)->vtype == TYPE_VECTOR)*/
+#define fold_immvalue_float(X)  ((X)->constval.vfloat)
+/*#define fold_immvalue_vector(X) ((X)->constval.vvec)*/
+/*#define fold_immvalue_string(X) ((X)->constval.vstring)*/
+#define fold_can_1(X)           ((X)->hasvalue && (X)->cvq == CV_CONST)
+/*#define fold_can_2(X,Y)         (fold_can_1(X) && fold_can_1(Y))*/
+
+
+int fold_cond(ir_value *condval, ast_function *func, ast_ifthen *branch) {
+    if (isfloat(condval) && fold_can_1(condval) && OPTS_OPTIMIZATION(OPTIM_CONST_FOLD_DCE)) {
+        ast_expression_codegen *cgen;
+        ir_block               *elide;
+        ir_value               *dummy;
+        bool                    istrue  = (fold_immvalue_float(condval) == 1.0f && branch->on_true);
+        bool                    isfalse = (fold_immvalue_float(condval) == 0.0f && branch->on_false);
+        ast_expression         *path    = (istrue)  ? branch->on_true  :
+                                          (isfalse) ? branch->on_false : NULL;
+        if (!path) {
+            /*
+             * no path to take implies that the evaluation is if(0) and there
+             * is no else block. so eliminate all the code.
+             */
+            ++opts_optimizationcount[OPTIM_CONST_FOLD_DCE];
+            return true;
+        }
+
+        if (!(elide = ir_function_create_block(ast_ctx(branch), func->ir_func, ast_function_label(func, ((istrue) ? "ontrue" : "onfalse")))))
+            return false;
+        if (!(*(cgen = path->codegen))((ast_expression*)path, func, false, &dummy))
+            return false;
+        if (!ir_block_create_jump(func->curblock, ast_ctx(branch), elide))
+            return false;
+        /*
+         * now the branch has been eliminated and the correct block for the constant evaluation
+         * is expanded into the current block for the function.
+         */
+        func->curblock = elide;
+        ++opts_optimizationcount[OPTIM_CONST_FOLD_DCE];
+        return true;
+    }
+    return -1; /* nothing done */
+}