cmd/compile: store constant floats using integer constants

x86 is better at storing constant ints than constant floats.
(It uses a constant directly in the instruction stream, instead of
loading it from a constant global memory.)

Noticed as part of #67957

Change-Id: I9b7b586ad8e0fe9ce245324f020e9526f82b209d
Reviewed-on: https://go-review.googlesource.com/c/go/+/592596
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/_gen/AMD64.rules

@@ -1583,6 +1583,9 @@
 (MOVSDstore [off] {sym} ptr (MOVQi2f val) mem) => (MOVQstore  [off] {sym} ptr val mem)
 (MOVSSstore [off] {sym} ptr (MOVLi2f val) mem) => (MOVLstore  [off] {sym} ptr val mem)
 
+(MOVSDstore [off] {sym} ptr (MOVSDconst [f]) mem) && f == f => (MOVQstore [off] {sym} ptr (MOVQconst [int64(math.Float64bits(f))]) mem)
+(MOVSSstore [off] {sym} ptr (MOVSSconst [f]) mem) && f == f => (MOVLstore [off] {sym} ptr (MOVLconst [int32(math.Float32bits(f))]) mem)
+
 // Load args directly into the register class where it will be used.
 // We do this by just modifying the type of the Arg.
 (MOVQf2i <t> (Arg <u> [off] {sym})) && t.Size() == u.Size() => @b.Func.Entry (Arg <t> [off] {sym})

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -227,3 +227,12 @@ func Float64DenormalFloat32Constant() float64 {
 	// ppc64x:"FMOVD\t[$]f64\\.3800000000000000\\(SB\\)"
 	return 0x1p-127
 }
+
+func Float64ConstantStore(p *float64) {
+	// amd64: "MOVQ\t[$]4617801906721357038"
+	*p = 5.432
+}
+func Float32ConstantStore(p *float32) {
+	// amd64: "MOVL\t[$]1085133554"
+	*p = 5.432
+}