[llvm-dev] LLVM Optimizations strange behavior/bug
Garba Peter via llvm-dev
llvm-dev at lists.llvm.org
Sun Apr 23 02:59:51 PDT 2017
What you see here is a 1:1 translation of RISC-V assembler code into C code.
It may contain some obfuscations like shifting by negative amount but this I why I use compiler optimization to remove such patterns.
But with the provided sample clang/llvm seems to be too optimistic with optimizations compare to GCC and CL and I really would like to
get this one fixed to use the output LLVM IR for further optimization on the optimized code and get the same behavior as the other compilers.
From: Tim Northover [mailto:t.p.northover at gmail.com]
Sent: Samstag, 22. April 2017 20:23
To: Garba Peter <peter.garba at gemalto.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] LLVM Optimizations strange behavior/bug
On 22 April 2017 at 04:18, Garba Peter via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Any idea if this is a bug or why clang does show this behavior ?
The code is pretty rife with undefined behaviour. Casting a "char *"
pointer to an "unsigned *" and dereferencing it violates strict aliasing (actually just doing the cast is dodgy, but usually not a problem in practice).
When I change those lines to use memcpy instead and compile with -fsanitize=undefined, apparently 4 of the shift operations are shifting by a negative amount (also undefined behaviour). I expect Clang is marking those as undef and simplifying everything down to a constant based on that.
Certainly I start getting non-constant results when I fix those. Also, beware that shifting a signed int is only valid if the input is positive and the result is still fits, and you can only shift from 0 to the 1 less than the bit-width of the type. Generally you almost always want to do bitwise fiddling on unsigned quantities because of that first one.
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