[dev.ssa] cmd/compile: cse should treat -0.0 and 0.0 as different

cse was incorrectly classifying -0.0 and 0.0 as equivalent. This lead
to invalid code as ssa uses PXOR -0.0, reg to negate a floating point.

Fixes math.

Change-Id: Id7eb10c71749eaed897f29b02c33891cf5820acf
Reviewed-on: https://go-review.googlesource.com/14205Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/TODO

@@ -44,6 +44,7 @@ Optimizations (better compiler)
 - Reuseable slices (e.g. []int of size NumValues()) cached in Func
 - Handle signed division overflow and sign extension earlier
 - Implement 64 bit const division with high multiply, maybe in the frontend?
+- Store bool and float32/float64 in auxInt
 
 Regalloc
 --------

src/cmd/compile/internal/ssa/cse.go

@@ -4,7 +4,10 @@
 
 package ssa
 
-import "sort"
+import (
+	"math"
+	"sort"
+)
 
 // cse does common-subexpression elimination on the Function.
 // Values are just relinked, nothing is deleted.  A subsequent deadcode
@@ -51,7 +54,19 @@ func cse(f *Func) {
 			if len(v.Args) > 1 {
 				arg1op = v.Args[1].Op
 			}
-			k := key{v.Op, v.Type.String(), v.Aux, v.AuxInt, len(v.Args), bid, arg0op, arg1op}
+
+			aux := v.Aux
+			auxInt := v.AuxInt
+			// -0 == 0, but aren't equivalent values so we use
+			// Float64bits to distinguish
+			if f, ok := aux.(float64); ok {
+				aux = nil
+				if auxInt != 0 {
+					v.Fatalf("float would clobber v.auxInt")
+				}
+				auxInt = int64(math.Float64bits(f))
+			}
+			k := key{v.Op, v.Type.String(), aux, auxInt, len(v.Args), bid, arg0op, arg1op}
 			m[k] = append(m[k], v)
 		}
 	}