runtime: always use 1MB stacks on 32-bit Windows

Commit c2c07c79 (CL 49331) changed the linker and runtime to always
use 2MB stacks on 64-bit Windows. This is the corresponding change to
make 32-bit Windows always use large (1MB) stacks because it's
difficult to detect when Windows applications will call into arbitrary
C code that may expect a large stack.

This is done as a separate change because it's possible this will
cause too much address space pressure for a 32-bit address space. On
the other hand, cgo binaries on Windows already use 1MB stacks and
there haven't been complaints.

Updates #20975.

Change-Id: I8ce583f07cb52254fb4bd47250f1ef2b789bc490
Reviewed-on: https://go-review.googlesource.com/49610
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/cmd/link/internal/ld/pe.go

@@ -857,14 +857,10 @@ func (f *peFile) writeOptionalHeader(ctxt *Link) {
 		oh64.SizeOfStackCommit = 0x00200000 - 0x2000 // account for 2 guard pages
 	}
 
-	// 32-bit is trickier since there much less address space to
-	// work with. Here we use large stacks only in cgo binaries as
-	// a compromise.
+	oh.SizeOfStackReserve = 0x00100000
 	if !iscgo {
-		oh.SizeOfStackReserve = 0x00020000
 		oh.SizeOfStackCommit = 0x00001000
 	} else {
-		oh.SizeOfStackReserve = 0x00100000
 		oh.SizeOfStackCommit = 0x00100000 - 0x2000 // account for 2 guard pages
 	}
 

src/runtime/os_windows.go

@@ -623,7 +623,7 @@ func semacreate(mp *m) {
 func newosproc(mp *m, stk unsafe.Pointer) {
 	const _STACK_SIZE_PARAM_IS_A_RESERVATION = 0x00010000
 	// stackSize must match SizeOfStackReserve in cmd/link/internal/ld/pe.go.
-	const stackSize = 0x00200000*_64bit + 0x00020000*(1-_64bit)
+	const stackSize = 0x00200000*_64bit + 0x00100000*(1-_64bit)
 	thandle := stdcall6(_CreateThread, 0, stackSize,
 		funcPC(tstart_stdcall), uintptr(unsafe.Pointer(mp)),
 		_STACK_SIZE_PARAM_IS_A_RESERVATION, 0)