cmd/loopvec-toolexec: rewrite default builds on darwin, ios, android and windows

cmd/loopvec-toolexec/compile.go

@@ -34,10 +34,30 @@ type compileArgs struct {
 	importcfg int   // index in args of the importcfg path, or -1
 	files     []int // indexes in args of the Go source files
 	// unsupported is set for build modes whose objects the nested simd build
-	// would not match (race, msan, asan, shared, dynlink, -trimpath).
+	// would not match (race, msan, asan, shared, dynlink, -trimpath). -shared
+	// is also what the go command passes for a position-independent
+	// executable, the default build mode on some targets (see defaultPIE);
+	// there the nested build uses the same default and matches.
 	unsupported bool
 }
 
+// defaultPIE reports whether the target builds position-independent
+// executables by default (internal/platform.DefaultPIE), so that the go command
+// passes -shared to every compile of a default build. The go command sets GOOS
+// for the tools it runs. On windows -race turns the default off, but -race is
+// unsupported here on its own.
+func defaultPIE() bool {
+	goos := os.Getenv("GOOS")
+	if goos == "" {
+		goos = runtime.GOOS
+	}
+	switch goos {
+	case "android", "darwin", "ios", "windows":
+		return true
+	}
+	return false
+}
+
 func parseCompileArgs(args []string) compileArgs {
 	c := compileArgs{importcfg: -1}
 	first := len(args)
@@ -59,8 +79,10 @@ func parseCompileArgs(args []string) compileArgs {
 			c.lang = args[i+1]
 		case strings.HasPrefix(arg, "-lang="):
 			c.lang = strings.TrimPrefix(arg, "-lang=")
-		case arg == "-race", arg == "-msan", arg == "-asan", arg == "-shared", arg == "-dynlink":
+		case arg == "-race", arg == "-msan", arg == "-asan", arg == "-dynlink":
 			c.unsupported = true
+		case arg == "-shared":
+			c.unsupported = !defaultPIE()
 		case arg == "-trimpath" && hasValue && strings.Contains(args[i+1], ";"):
 			c.unsupported = true
 		}

testdata/toolexec_buildmode_pie.txt

@@ -0,0 +1,64 @@
+# The go command compiles a position-independent executable with -shared. It is the
+# default build mode on darwin, ios, android and windows, where the nested simd build
+# is one too, so a default build is rewritten there. Asking for one where it is not
+# the default (-buildmode=pie on linux) still leaves the package alone: the nested
+# build would not match it.
+
+# Reference: the scalar program.
+exec go run .
+cmp stdout want.out
+
+env GOEXPERIMENT=simd
+env GOCACHE=$WORK/gocache
+env LOOPVEC_TOOLEXEC_LOG=$WORK/log
+env LOOPVEC_TOOLEXEC_LOG_PKGS=example.com/app/vec
+
+[GOOS:darwin] exec go build -toolexec=$LOOPVEC_TOOLEXEC -o app .
+[GOOS:darwin] cmp log rewrote.want
+
+[GOOS:linux] env LOOPVEC_TOOLEXEC_DEBUG=1
+[GOOS:linux] exec go build -buildmode=pie -toolexec=$LOOPVEC_TOOLEXEC -o app .
+[GOOS:linux] cmp log skipped.want
+
+[GOOS:darwin] exec ./app
+[GOOS:darwin] cmp stdout want.out
+[GOOS:linux] exec ./app
+[GOOS:linux] cmp stdout want.out
+
+-- go.mod --
+module example.com/app
+
+go 1.27.1
+-- vec/vec.go --
+package vec
+
+func AddInt32s(dst, a, b []int32) {
+	for i := range dst {
+		dst[i] = a[i] + b[i]
+	}
+}
+-- main.go --
+package main
+
+import (
+	"fmt"
+
+	"example.com/app/vec"
+)
+
+func main() {
+	dst := make([]int32, 21)
+	a := make([]int32, 21)
+	b := make([]int32, 21)
+	for i := range a {
+		a[i], b[i] = int32(i), int32(2*i)
+	}
+	vec.AddInt32s(dst, a, b)
+	fmt.Println(dst)
+}
+-- want.out --
+[0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60]
+-- rewrote.want --
+rewrote 1 loop(s) in example.com/app/vec
+-- skipped.want --
+skipped example.com/app/vec: simd unavailable or unsupported build mode

toolexec.md

@@ -28,7 +28,9 @@ Packages are left alone when:
   `os`, `strconv`, `sync`, `sync/atomic`, `math`, and more). Rewritten code
   imports `simd`, so rewriting these would create an import cycle.
 - They use cgo, or the build uses `-race`, `-msan`, `-asan`, `-shared`,
-  `-dynlink` or `-trimpath`.
+  `-dynlink` or `-trimpath`. `-shared` is accepted where a position-independent
+  executable is the default build (darwin, ios, android, windows), since the
+  nested simd build is one there too.
 - They are cryptographic: `crypto/...` and `golang.org/x/crypto/...`, vendored
   copies included. Constant-time code is written so that its running time does
   not depend on secret data, and a rewrite adds a runtime overlap check, a length