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accel/tcg: Add URL of clang bug to comment about our workaround 

In cpu_exec() we have a longstanding workaround for compilers which
do not correctly implement the part of the sigsetjmp()/siglongjmp()
spec which requires that local variables which are not changed
between the setjmp and the longjmp retain their value.

I recently ran across the upstream clang bug report for this; add a
link to it to the comment describing the workaround, and generally
expand the comment, so that we have a reasonable chance in future of
understanding why it's there and determining when we can remove it,
assuming clang eventually fixes the bug.

Remove the /* buggy compiler */ comments on the #else and #endif:
they don't add anything to understanding and are somewhat misleading
since they're sandwiching the code path for *non*-buggy compilers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20210129130330.30820-1-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2021-01-29 13:03:30 +00:00
parent 932a8d1f11
commit e6a41a045c
1 changed files with 19 additions and 6 deletions
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25
accel/tcg/cpu-exec.c
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@ -773,17 +773,30 @@ int cpu_exec(CPUState *cpu)
/* prepare setjmp context for exception handling */
if (sigsetjmp(cpu->jmp_env, 0) != 0) {
#if defined(__clang__)
/* Some compilers wrongly smash all local variables after
* siglongjmp. There were bug reports for gcc 4.5.0 and clang.
/*
* Some compilers wrongly smash all local variables after
* siglongjmp (the spec requires that only non-volatile locals
* which are changed between the sigsetjmp and siglongjmp are
* permitted to be trashed). There were bug reports for gcc
* 4.5.0 and clang. The bug is fixed in all versions of gcc
* that we support, but is still unfixed in clang:
* https://bugs.llvm.org/show_bug.cgi?id=21183
*
* Reload essential local variables here for those compilers.
* Newer versions of gcc would complain about this code (-Wclobbered). */
* Newer versions of gcc would complain about this code (-Wclobbered),
* so we only perform the workaround for clang.
*/
cpu = current_cpu;
cc = CPU_GET_CLASS(cpu);
#else /* buggy compiler */
/* Assert that the compiler does not smash local variables. */
#else
/*
* Non-buggy compilers preserve these locals; assert that
* they have the correct value.
*/
g_assert(cpu == current_cpu);
g_assert(cc == CPU_GET_CLASS(cpu));
#endif /* buggy compiler */
#endif
#ifndef CONFIG_SOFTMMU
tcg_debug_assert(!have_mmap_lock());
#endif