reflect: avoid a bounds check in stack-constrained code

Since CL 682496 we need more stack space to handle bounds checks.
The code modified here normally has no bounds checks, but in -N
builds it still does and thus uses too much stack.

Use unsafe arithmetic to avoid the bounds check.

This will hopefully fix some of the arm64 linux builders.

Change-Id: I5b3096a14b4fb9553e635b7f340e60b8ffba8755
Reviewed-on: https://go-review.googlesource.com/c/go/+/690415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/reflect/makefunc.go

@@ -8,6 +8,7 @@ package reflect
 
 import (
 	"internal/abi"
+	"internal/goarch"
 	"unsafe"
 )
 
@@ -164,13 +165,18 @@ func moveMakeFuncArgPtrs(ctxt *makeFuncCtxt, args *abi.RegArgs) {
 	for i, arg := range args.Ints {
 		// Avoid write barriers! Because our write barrier enqueues what
 		// was there before, we might enqueue garbage.
+		// Also avoid bounds checks, we don't have the stack space for it.
+		// (Normally the prove pass removes them, but for -N builds we
+		// use too much stack.)
+		// ptr := &args.Ptrs[i] (but cast from *unsafe.Pointer to *uintptr)
+		ptr := (*uintptr)(add(unsafe.Pointer(unsafe.SliceData(args.Ptrs[:])), uintptr(i)*goarch.PtrSize, "always in [0:IntArgRegs]"))
 		if ctxt.regPtrs.Get(i) {
-			*(*uintptr)(unsafe.Pointer(&args.Ptrs[i])) = arg
+			*ptr = arg
 		} else {
 			// We *must* zero this space ourselves because it's defined in
 			// assembly code and the GC will scan these pointers. Otherwise,
 			// there will be garbage here.
-			*(*uintptr)(unsafe.Pointer(&args.Ptrs[i])) = 0
+			*ptr = 0
 		}
 	}
 }