[llvm-commits] [llvm] r126964 - in /llvm/trunk: lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp lib/CodeGen/SelectionDAG/LegalizeTypes.h test/CodeGen/X86/umulo-64.ll

[Erik Olofsson]

Check out hackers delight. It usually has trickery for cases like this. Google:
hacker's delight check for multiplication overflow

Regards,
Erik

> -----Original Message-----
> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
> bounces at cs.uiuc.edu] On Behalf Of Eric Christopher
> Sent: den 4 mars 2011 09:29
> To: Evan Cheng
> Cc: llvm-commits at cs.uiuc.edu
> Subject: Re: [llvm-commits] [llvm] r126964 - in /llvm/trunk:
> lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
> lib/CodeGen/SelectionDAG/LegalizeTypes.h test/CodeGen/X86/umulo-64.ll
>
>
> On Mar 4, 2011, at 12:16 AM, Evan Cheng wrote:
>
> >
> >
> > On Mar 3, 2011, at 4:16 PM, Eric Christopher <echristo at apple.com> wrote:
> >
> >>
> >>> Err, I'm reasonably sure the code has precisely the same issues for
> >>> 64-bit... narrow my previous example and compile on x86-32, and you
> >>> end up with the following, which has the exact same issue:
> >>
> >> So I was trying to be clever and have no idea what I was thinking. :)
> >>
> >> Here's a simple patch that fixes the problem in something resembling
> correctness, but uses divide to do it - so yeah, performance?
> >>
> >
> > Math is hard.
>
> Yep.
>
> > But surely it's possible to generate better code. What do other compilers
> do?
>
> Not sure what anything other than gcc does. For gcc they kind of avoid some
> of the problem we're trying to solve here. If -ftrapv is called they use a
> libcall that aborts if the multiply overflows. Otherwise they just use straight
> mul. This probably won't work for us because one of the big places that
> clang uses this built-in is to check whether or not an allocation overflows so
> that they can throw an exception for c++. And yes, I'd rather not have to split
> the block for every allocation just to make an "is this zero" check before
> doing a divide. Especially since it'd be always false in the normal case.
>
> I'm uncertain what a more modern gcc does in the case that they actually
> want to do something with the overflow flag.
>
> -eric
>
>
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