fast-math: Fold fp * -1 to -fp (#3189)

Author: MaxGraey

src/passes/OptimizeInstructions.cpp

@@ -1447,8 +1447,7 @@ struct OptimizeInstructions
     }
     {
       double value;
-      if (fastMath &&
-          matches(curr, binary(Abstract::Sub, any(), fval(&value))) &&
+      if (matches(curr, binary(Abstract::Sub, any(), fval(&value))) &&
           value == 0.0) {
         // x - (-0.0)   ==>   x + 0.0
         if (std::signbit(value)) {
@@ -1470,6 +1469,10 @@ struct OptimizeInstructions
         return curr->left;
       }
     }
+    // x * -1.0   ==>   -x
+    if (fastMath && matches(curr, binary(Abstract::Mul, any(), fval(-1.0)))) {
+      return builder.makeUnary(Abstract::getUnary(type, Abstract::Neg), left);
+    }
     if (matches(curr, binary(Abstract::Mul, any(&left), constant(1))) ||
         matches(curr, binary(Abstract::DivS, any(&left), constant(1))) ||
         matches(curr, binary(Abstract::DivU, any(&left), constant(1)))) {

test/passes/O_fast-math.txt

@@ -1,5 +1,7 @@
 (module
  (type $none_=>_f32 (func (result f32)))
+ (type $f32_=>_f32 (func (param f32) (result f32)))
+ (type $f64_=>_f64 (func (param f64) (result f64)))
  (export "div" (func $0))
  (export "mul1" (func $1))
  (export "mul2" (func $2))
@@ -9,6 +11,8 @@
  (export "add4" (func $2))
  (export "sub1" (func $1))
  (export "sub2" (func $2))
+ (export "mul_neg_one1" (func $9))
+ (export "mul_neg_one2" (func $10))
  (func $0 (; has Stack IR ;) (result f32)
   (f32.const -nan:0x23017a)
  )
@@ -18,4 +22,14 @@
  (func $2 (; has Stack IR ;) (result f32)
   (f32.const -nan:0x74546d)
  )
+ (func $9 (; has Stack IR ;) (param $0 f32) (result f32)
+  (f32.neg
+   (local.get $0)
+  )
+ )
+ (func $10 (; has Stack IR ;) (param $0 f64) (result f64)
+  (f64.neg
+   (local.get $0)
+  )
+ )
 )

test/passes/O_fast-math.wast

@@ -54,4 +54,16 @@
    (f32.const -0)
   )
  )
+ (func "mul_neg_one1" (param $x f32) (result f32)
+  (f32.mul
+   (local.get $x)
+   (f32.const -1)
+  )
+ )
+ (func "mul_neg_one2" (param $x f64) (result f64)
+  (f64.mul
+   (local.get $x)
+   (f64.const -1)
+  )
+ )
 )

test/passes/optimize-instructions_all-features.txt

@@ -3829,15 +3829,15 @@
    )
   )
   (drop
-   (f32.sub
+   (f32.add
     (local.get $fx)
-    (f32.const -0)
+    (f32.const 0)
    )
   )
   (drop
-   (f64.sub
+   (f64.add
     (local.get $fy)
-    (f64.const -0)
+    (f64.const 0)
    )
   )
   (drop