[llvm-dev] The value of padding when storing an aggregate into memory

[Juneyoung Lee via llvm-dev]

Hello Alexander,

> Interesting topic. Is any such optimization reachable from C?
Yes, I think so - both PassBuilder and PassManagerBuilder add MemCpyOpt &
IPSCCP in the default pass pipeline.

Juneyoung

On Wed, Aug 19, 2020 at 8:43 PM Alexander Cherepanov <ch3root at openwall.com>
wrote:

> On 19/08/2020 06.05, Juneyoung Lee via llvm-dev wrote:
> > LangRef isn't clear about the value of padding when an aggregate value is
> > stored into memory, and I'd like to suggest that storing an aggregate
> fills
> > padding with undef.
> >
> > Here are a few clues that supports this change:
> >
> > - According to C17, the value of padding bytes when storing values in
> > structures or unions is unspecified.
> >
> > - IPSCCP ignores padding and directly stores a constant aggregate if
> > possible: https://godbolt.org/z/ddWq9z
> > Memcpyopt ignores padding when copying an aggregate or storing a
> constant:
> > https://godbolt.org/z/hY6ndd / https://godbolt.org/z/3WMP5a
>
> Interesting topic. Is any such optimization reachable from C?
>
> --
> Alexander Cherepanov
>


-- 

Juneyoung Lee
Software Foundation Lab, Seoul National University
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