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Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of sign
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values |
Date: |
Wed, 25 Nov 2015 18:50:03 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 25/11/2015 18:44, Peter Maydell wrote:
> > Ubsan also has warnings for undefined behavior of left shifts. Checks for
> > left shift overflow and left shift of negative numbers, unfortunately,
> > cannot be silenced without also silencing the useful ones about out-of-range
> > shift amounts. -fwrapv ought to shut them up, but doesn't yet
> > (https://llvm.org/bugs/show_bug.cgi?id=25552; I am taking care of fixing
> > the same issues in GCC). Luckily ubsan is optional, and the easy
> > workaround is to use -fsanitize-recover.
>
> We still haven't had any response from the LLVM/clang folks that
> this interpretation of the meaning of -fwrapv is their interpretation
> of it, have we? (I can't see any comments on the referenced bug.)
Reasonably, they will have to follow what GCC does, independent of
-fwrapv. GCC has now promised to not exploit << undefined behavior,
even without -fwrapv.
So at this point, -fwrapv is only required to placate ubsan---which it
will do for GCC as soon as my other patch is approved (I talked on IRC
with one of the GCC-ubsan authors and he said he was okay). clang with
ubsan remains broken, but that's no worse than before.
Paolo
- [Qemu-devel] [PULL 0/9] Misc patches for QEMU 2.5-rc2 (2015-11-25), Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 3/9] call bdrv_drain_all() even if the vm is stopped, Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 4/9] Revert "exec: silence hugetlbfs warning under qtest", Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 6/9] target-sparc: fix 32-bit truncation in fpackfix, Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 5/9] exec: remove warning about mempath and hugetlbfs, Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 8/9] target-i386: kvm: Use env->mcg_cap when setting up MCE, Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 1/9] MAINTAINERS: Update TCG CPU cores section, Paolo Bonzini, 2015/11/25
- [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values,
Paolo Bonzini <=
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/25
- Re: [Qemu-devel] [PULL 2/9] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/25
[Qemu-devel] [PULL 7/9] target-i386: kvm: Abort if MCE bank count is not supported by host, Paolo Bonzini, 2015/11/25
[Qemu-devel] [PULL 9/9] target-i386: kvm: Print warning when clearing mcg_cap bits, Paolo Bonzini, 2015/11/25
Re: [Qemu-devel] [PULL 0/9] Misc patches for QEMU 2.5-rc2 (2015-11-25), Peter Maydell, 2015/11/26