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

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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.

Backports e6a41a045c298538d303cd8fe8d7ae29a0c066ad
This commit is contained in:
Peter Maydell 2021-03-04 18:32:42 -05:00 committed by Lioncash
parent 93b0428f48
commit 2c926832bb
1 changed files with 18 additions and 6 deletions
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24
qemu/accel/tcg/cpu-exec.c
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@ -551,18 +551,30 @@ int cpu_exec(struct uc_struct *uc, CPUState *cpu)
/* prepare setjmp context for exception handling */ /* prepare setjmp context for exception handling */
if (sigsetjmp(cpu->jmp_env, 0) != 0) { if (sigsetjmp(cpu->jmp_env, 0) != 0) {
#if defined(__clang__) #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. * 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 = uc->current_cpu; cpu = uc->current_cpu;
env = cpu->env_ptr; env = cpu->env_ptr;
cc = CPU_GET_CLASS(uc, cpu); cc = CPU_GET_CLASS(uc, cpu);
#else /* buggy compiler */ #else
/* Assert that the compiler does not smash local variables. */ /*
* Non-buggy compilers preserve these locals; assert that
* they have the correct value.
*/
g_assert(cpu == uc->current_cpu); g_assert(cpu == uc->current_cpu);
g_assert(cc == CPU_GET_CLASS(uc, cpu)); g_assert(cc == CPU_GET_CLASS(uc, cpu));
#endif /* buggy compiler */ #endif
// Unicorn: commented out // Unicorn: commented out
//tb_lock_reset(); //tb_lock_reset();
} }