[libunwind] Use __builtin_alloca to avoid missing include

[ldionne]

Summary

This is required for picolibc. picolibc (possibly erroneously) only includes alloca.h from stdlib.h if STRICT_ANSI is not defined - see https://github.com/picolibc/picolibc/blob/fe20d30008df0f3bb334a202bea344b8a08d32e7/newlib/libc/include/stdlib.h#L54 . This patch includes alloca.h by default.

[barannikov88]

picolibc (possibly erroneously) only includes alloca.h from stdlib.h if STRICT_ANSI is not defined

The header is non-standard. It is correct not to include it when using -std=c* (as opposed to -std=gnu*).
If libunwind relies on it in public header files, it should be rectified.

[mstorsjo]

In D149635#4311710, @barannikov88 wrote:

picolibc (possibly erroneously) only includes alloca.h from stdlib.h if STRICT_ANSI is not defined

The header is non-standard. It is correct not to include it when using -std=c* (as opposed to -std=gnu*).
If libunwind relies on it in public header files, it should be rectified.

Indeed. In particular, mingw toolchains don't have an alloca.h header - this should show up as failure from those jobs in CI.

[danielkiss]

libunwind/src/config.h

130

alloca.h is needed for this so probably the simplest way is just to build with _LIBUNWIND_REMEMBER_HEAP_ALLOC if that causes trouble for you.

[xbjfk]

Indeed. In particular, mingw toolchains don't have an alloca.h header - this should show up as failure from those jobs in CI.

I see, maybe in this case alloca.h should only be included in the #ifndef _LIBUNWIND_REMEMBER_HEAP_ALLOC and under __has_include? not sure whether this is acceptable.
or is this problem something for picolibc to solve? or the user by specifying -include alloca.h? how should this be fixed instead?

[mstorsjo]

In D149635#4311953, @xbjfk wrote:

Indeed. In particular, mingw toolchains don't have an alloca.h header - this should show up as failure from those jobs in CI.

I see, maybe in this case alloca.h should only be included in the #ifndef _LIBUNWIND_REMEMBER_HEAP_ALLOC and under __has_include? not sure whether this is acceptable.
or is this problem something for picolibc to solve? or the user by specifying -include alloca.h? how should this be fixed instead?

Well I guess technically libunwind is wrong here; if we do use alloca, we should include a corresponding header. The tricky thing is just that it's rather platform specific how to do that; see e.g. https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0/openmp/runtime/src/kmp_wrapper_malloc.h#L93-L104 for one case of such an ifdef jungle.

[compnerd]

libunwind/src/config.h

16

Is this available on MSVC?

[mstorsjo]

libunwind/src/config.h

16

It's not, as already commented above - it's not available in either MSVC or mingw toolchains.

[compnerd]

libunwind/src/config.h

16

Should this be something that we put into a configuration then? Or provide shims for?

[compnerd]

libunwind/src/config.h

130

Sure, that makes sense, but the inclusion of the header should be guarded by _LIBUNWIND_REMEMBER_HEAP_ALLOC then right?

[arichardson]

libunwind/src/config.h

16

Could we avoid the header and just use __builtin_alloca? Or is that code path used with compilers that don't support this built-in?

[ldionne]

libunwind/src/config.h

16

+1, let's use __builtin_alloca if we can and drop the non-portable include.

[barannikov88]

libunwind/src/config.h

16

+1, also thought about it. But is it available on MSVC? Or libunwind is not supposed to be compiled by MSVC?

[ldionne]

[Github PR transition cleanup]

Commandeering to finish.

[ldionne]

Use __builtin_alloca

[ldionne]

Rebase and poke CI.

[danielkiss]

libunwind/src/config.h

129

maybe we could add an extra guard just to be sure

[ldionne]

libunwind/src/config.h

129

Honestly, I'd rather see it fail explicitly in that case than silently fall back to using malloc. If someone uses a compiler we don't support here, the correct path IMO would be for it to fail fast and loud (at compile-time).

[ldionne]

Shipping since the CI issue is a fluke.

libunwind/src/config.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //
 //  Defines macros used within libunwind project.
 //
 //===----------------------------------------------------------------------===//
 
 
 #ifndef LIBUNWIND_CONFIG_H
 #define LIBUNWIND_CONFIG_H
 
 #include <assert.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>
 
 #include <__libunwind_config.h>
 
 // Platform specific configuration defines.
 #ifdef __APPLE__

 #if !defined(_LIBUNWIND_BUILD_SJLJ_APIS)
 #define _LIBUNWIND_BUILD_ZERO_COST_APIS
 #endif
 #endif
 
 #ifndef _LIBUNWIND_REMEMBER_HEAP_ALLOC
 #if defined(_LIBUNWIND_REMEMBER_STACK_ALLOC) || defined(__APPLE__) ||          \
     defined(__linux__) || defined(__ANDROID__) || defined(__MINGW32__) ||      \
     defined(_LIBUNWIND_IS_BAREMETAL)
-#define _LIBUNWIND_REMEMBER_ALLOC(_size) alloca(_size)
+#define _LIBUNWIND_REMEMBER_ALLOC(_size) __builtin_alloca(_size)
 #define _LIBUNWIND_REMEMBER_FREE(_ptr)                                         \
   do {                                                                         \
   } while (0)
 #elif defined(_WIN32)
 #define _LIBUNWIND_REMEMBER_ALLOC(_size) _malloca(_size)
 #define _LIBUNWIND_REMEMBER_FREE(_ptr) _freea(_ptr)
 #define _LIBUNWIND_REMEMBER_CLEANUP_NEEDED
 #else