cmd/compile/internal/mips64: fix large uint -> float conversion

Re-enable TestFP in cmd/compile/internal/gc on mips64.

Fixes #15552.

Change-Id: I5c3a5564b94d28c723358f0862468fb6da371991
Reviewed-on: https://go-review.googlesource.com/22835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa_test.go

@@ -57,12 +57,7 @@ func TestArithmetic(t *testing.T) {
 }
 
 // TestFP tests that both backends have the same result for floating point expressions.
-func TestFP(t *testing.T) {
-	if runtime.GOARCH == "mips64" || runtime.GOARCH == "mips64le" {
-		t.Skip("legacy mips64 compiler doesn't handle uint->float conversion correctly (issue 15552)")
-	}
-	runTest(t, "fp_ssa.go")
-}
+func TestFP(t *testing.T) { runTest(t, "fp_ssa.go") }
 
 // TestArithmeticBoundary tests boundary results for arithmetic operations.
 func TestArithmeticBoundary(t *testing.T) { runTest(t, "arithBoundary_ssa.go") }

src/cmd/compile/internal/mips64/gsubr.go

@@ -466,7 +466,7 @@ func gmove(f *gc.Node, t *gc.Node) {
 	//return;
 	// algorithm is:
 	//	if small enough, use native int64 -> float64 conversion.
-	//	otherwise, halve (rounding to odd?), convert, and double.
+	//	otherwise, halve (x -> (x>>1)|(x&1)), convert, and double.
 	/*
 	 * integer to float
 	 */
@@ -496,9 +496,16 @@ func gmove(f *gc.Node, t *gc.Node) {
 			gmove(&bigi, &rtmp)
 			gins(mips.AAND, &r1, &rtmp)
 			p1 := ginsbranch(mips.ABEQ, nil, &rtmp, nil, 0)
-			p2 := gins(mips.ASRLV, nil, &r1)
+			var r3 gc.Node
+			gc.Regalloc(&r3, gc.Types[gc.TUINT64], nil)
+			p2 := gins3(mips.AAND, nil, &r1, &r3)
 			p2.From.Type = obj.TYPE_CONST
 			p2.From.Offset = 1
+			p3 := gins(mips.ASRLV, nil, &r1)
+			p3.From.Type = obj.TYPE_CONST
+			p3.From.Offset = 1
+			gins(mips.AOR, &r3, &r1)
+			gc.Regfree(&r3)
 			gc.Patch(p1, gc.Pc)
 		}