cmd/go: add GOMIPS value to build id for mipsle

Strip a trailing "le" from the GOARCH value when calculating the GOxxx
environment variable that affects it.

Fixes #27260

Change-Id: I081f30d5dc19281901551823f4f56be028b5f71a
Reviewed-on: https://go-review.googlesource.com/131379Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/work/exec.go

@@ -224,7 +224,9 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		if len(p.SFiles) > 0 {
 			fmt.Fprintf(h, "asm %q %q %q\n", b.toolID("asm"), forcedAsmflags, p.Internal.Asmflags)
 		}
-		fmt.Fprintf(h, "GO$GOARCH=%s\n", os.Getenv("GO"+strings.ToUpper(cfg.BuildContext.GOARCH))) // GO386, GOARM, etc
+		// GO386, GOARM, GOMIPS, etc.
+		baseArch := strings.TrimSuffix(cfg.BuildContext.GOARCH, "le")
+		fmt.Fprintf(h, "GO$GOARCH=%s\n", os.Getenv("GO"+strings.ToUpper(baseArch)))
 
 		// TODO(rsc): Convince compiler team not to add more magic environment variables,
 		// or perhaps restrict the environment variables passed to subprocesses.

src/cmd/go/testdata/script/build_cache_gomips.txt

+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Building for mipsle without setting GOMIPS will use floating point registers.
+env GOARCH=mipsle
+env GOOS=linux
+go build -gcflags=-S f.go
+stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Clean cache
+go clean -cache
+
+# Building with GOMIPS=softfloat will not use floating point registers
+env GOMIPS=softfloat
+go build -gcflags=-S f.go
+! stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Clean cache
+go clean -cache
+
+# Build without setting GOMIPS
+env GOMIPS=
+go build -gcflags=-S f.go
+stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Building with GOMIPS should still not use floating point registers.
+env GOMIPS=softfloat
+go build -gcflags=-S f.go
+! stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+-- f.go --
+package f
+
+func F(x float64) float64 {
+     return x + x
+}