[llvm-commits] [PATCH] [MIPS] Avoid use of __builtin_unreachable() when compiling LLVM for MIPS.

[Petar Jovanovic]

The issue was first found on the latest cross gcc from Mentor [gcc version
4.5.2 (Sourcery CodeBench Lite 2011.09-75)], and since
__builtin_unreachable() is available as of gcc 4.5, it seemed to me that the
scope is already narrow enough. I can also confirm that the bug can be
reproduced with native gcc (GCC) 4.5.1 and gcc (GCC) 4.5.3 , and I am
actually not aware of gcc version that supports __builtin_unreachable in
which the bug is not present.

The bug was discovered three days ago in Android builds for MIPS. The
function where this issue was discovered is: 
unsigned TargetData::getAlignment(Type *Ty, bool abi_or_pref)

at lib\Target\TargetData.cpp.

Petar

-----Original Message-----
From: Benjamin Kramer [mailto:benny.kra at googlemail.com] 
Sent: Friday, March 23, 2012 12:57 AM
To: Petar Jovanovic
Cc: llvm-commits at cs.uiuc.edu; Petar Jovanovic
Subject: Re: [llvm-commits] [PATCH] [MIPS] Avoid use of
__builtin_unreachable() when compiling LLVM for MIPS.

On 23.03.2012, at 00:20, Petar Jovanovic <petar.jovanovic at rt-rk.com> wrote:

> There is a GCC bug which can be triggered inside of LLVM library. The -O2
opt
> does not work well with __builtin_unreachable(), so we are avoiding it
inside
> of LLVM. The issue seems to be caused by 'freorder-blocks' optimization.

Patches working around compiler bugs are generally okay, but please

- Add a comment on why you're blacklisting MIPS here.
- Make the blacklist as narrow as possible, i.e. check for the
specific version of GCC that has this bug.

- Ben
>
> Signed-off-by: Petar Jovanovic <petarj at mips.com>
> ---
> include/llvm/Support/Compiler.h |    2 ++
> 1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/include/llvm/Support/Compiler.h
b/include/llvm/Support/Compiler.h
> index 9c5e7ec..b5a086a 100644
> --- a/include/llvm/Support/Compiler.h
> +++ b/include/llvm/Support/Compiler.h
> @@ -144,9 +144,11 @@
> // LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
> // to an expression which states that it is undefined behavior for the
> // compiler to reach this point.  Otherwise is not defined.
> +#if !defined(__mips__)
> #if defined(__clang__) || (__GNUC__ > 4) \
>  || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
> # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
> #endif
> +#endif
>
> #endif
> --
> 1.7.5.4
>
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