Fix "#pragma clang __debug overflow_stack" for MSVC builds

[gbedwell]

Summary

Hi Clang Developers,

I noticed that using "#pragma clang __debug overflow_stack" on clang.exe built by Visual Studio 2012, to force a stack overflow causes it to seemingly hang indefinitely (I left it running overnight and it hadn't finished by morning). This patch just ensures that there is actually some new stack usage on every call to DebugOverflowStack. With this patch the overflow_stack pragma causes clang.exe to crash immediately with a big callstack full of DebugOverflowStack as I'd expect.

I've got a test for it, but when testing on Linux ran into the issue described at https://savannah.gnu.org/bugs/?22010 where GNU Make 3.81 has a bug where spawned processes get an unlimited stack size. Obviously this doesn't play well with a test designed to overflow the stack and brought the whole system to a halt. It works correctly when running through lit directly.

Here is my test for reference. Any suggestions to robustify it against the above behaviour are appreciated! I thought about adding a "REQUIRES: system-windows" but I'm not sure if some people may be using Make on Windows or whether it suffers from the same issue as on Linux.

---

RUN: not %clang %s -fsyntax-only 2>&1 | FileCheck %s
REQUIRES: crash-recovery

#pragma clang __debug overflow_stack

// CHECK: PLEASE submit a bug report

Otherwise, is it okay to commit the attached patch without the test?

Many Thanks,

Greg Bedwell
SN Systems - Sony Computer Entertainment Inc.

[gbedwell]

...and right as I hit submit I realised I submitted my non-clang-formatted version, but Phabricator is not letting me submit an updated version at the moment. d'oh!

obviously should look like this:

void *StackUser = &StackUser;

[rsmith]

lib/Lex/Pragma.cpp

928–932

A smart compiler could still optimize out the stack usage here. How about using something that keeps all the stack memory live, such as:

volatile void *Head;
static void DebugOverflowStack(void *Curr = 0) {
  Head = Curr;
  DebugOverflowStack(&Curr);
}

Hopefully MSVC doesn't optimize out stores to volatile globals...

[rsmith]

lib/Lex/Pragma.cpp

928–932

Sorry, should be

void *volatile Head;

[• rafael]

There is a FIXME in test/Makefile about moving the ULIMIT setting from the Makefile to lit. The Make bug you found is another reason to do it, but that can be in another patch.

lib/Lex/Pragma.cpp

929

Add a comment explaining what this is.

[Eugene.Zelenko]

Looks like patch was not committed. Does it still make sense?

lib/Lex/Pragma.cpp

void HandleCaptured(Preprocessor &PP) {
     PP.EnterTokenStream(Toks, 1, /*DisableMacroExpansion=*/true,
                         /*OwnsTokens=*/false);
   }
 
 // Disable MSVC warning about runtime stack overflow.
 #ifdef _MSC_VER
     #pragma warning(disable : 4717)
 #endif
   static void DebugOverflowStack() {
+    void* StackUser = &StackUser;
     void (*volatile Self)() = DebugOverflowStack;
     Self();
   }
 #ifdef _MSC_VER
     #pragma warning(default : 4717)
 #endif
 
 };
 
 /// PragmaDiagnosticHandler - e.g. '\#pragma GCC diagnostic ignored "-Wformat"'
 struct PragmaDiagnosticHandler : public PragmaHandler {