[llvm-dev] Which pass should be propagating memory copies

[Keno Fischer via llvm-dev]

Hi Daniel,

as far as I can tell that handles turning small memcpys into store
instructions. What I'm looking for
is something that can simplify (copy to stack) -> (modify stack) -> (copy
back to heap) into a single
heap modification.

Keno

On Tue, May 16, 2017 at 1:50 PM, Daniel Neilson <dneilson at azul.com> wrote:

>  The InstCombine transform does exactly what you want.  Take a look at
> lib/Transforms/Scalar/InstCombine/InstCombineCalls.cpp — InstCombiner::
> SimplifyMemTransfer
>
>  With your align parameter on the memcpy being zero you are likely hitting
> the first conditional in that function:
>   if (CopyAlign < MinAlign) {
>     MI->setAlignment(ConstantInt::get(MI->getAlignmentType(), MinAlign,
> false));
>     return MI;
>   }
>
>  Arguably, instcombine probably shouldn’t bail on trying to simplify the
> memcpy just because it could update the alignment on the call...
>
> -Daniel
>
> > On May 16, 2017, at 12:37 PM, Keno Fischer via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Consider the following IR example:
> >
> > define void @simple([4 x double] *%ptr, i64 %idx) {
> >     %stack = alloca [4 x double]
> >     %ptri8 = bitcast [4 x double] *%ptr to i8*
> >     %stacki8 = bitcast [4 x double] *%stack to i8*
> >     call void @llvm.memcpy.p0i8.p0i8.i32(i8 *%stacki8, i8 *%ptri8, i32
> 32, i32 0, i1 0)
> >     %dataptr = getelementptr inbounds [4 x double], [4 x double] *%ptr,
> i32 0, i64 %idx
> >     store double 0.0, double *%dataptr
> >     call void @llvm.memcpy.p0i8.p0i8.i32(i8 *%ptri8, i8 *%stacki8, i32
> 32, i32 0, i1 0)
> >     ret void
> > }
> >
> >
> > I would like to see this optimized to just a single store (into %ptr).
> Right now, even at -O3 that doesn't happen. My frontend guarantees that idx
> is always inbounds for the allocation, but I do think the transformation
> should be valid regardless because accessing beyond the bounds of the
> alloca should be undefined behavior. Now, my question is which pass should
> be responsible for doing this? SROA? DSE? GVN? A new pass just to do this
> kind of thing? Maybe there already is some pass that does this, just not in
> the default pipeline? Any hints would be much appreciated.
> >
> > Thanks,
> > Keno
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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