Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/11/2015 20:18, Peter Maydell wrote:
> And LLVM is its own project and its developers don't always exactly
> follow gcc behaviour.

True.  The patch docuuments that if LLVM will not respect 2s complement
for signed shifts when passed the -fwrapv option, it will not be
supported for compilation of QEMU.  But let;s cross that bridge when we
reach it.  So far, -Wshift-negative-value seems to be a misguided
attempt to copy GCC's warning without understanding it.

> Your latest patch at https://gcc.gnu.org/ml/gcc-patches/2015-11/msg03055.html
> doesn't seem to touch the documentation of -fwrapv at all, so I
> don't think it is sufficient to allow users of the compiler
> to say "-fwrapv means signed behaviour for shifts".

GCC *always* does signed behavior for shifts, even without -fwrapv.
I'll commit tomorrow the patch that promises that for the future.

GCC does not need -fwrapv at all.

Paolo