[PATCH] D114357: [CodeGen][AArch64] Ensure isSExtCheaperThanZExt returns true for negative constants
David Sherwood via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 18 00:39:06 PST 2022
david-arm added a comment.
In D114357#3250506 <https://reviews.llvm.org/D114357#3250506>, @mstorsjo wrote:
> I've bisected a miscompilation to this file.
>
> To reproduce:
>
> $ git clone git://source.ffmpeg.org/ffmpeg
> $ cd ffmpeg
> $ ./configure --cc=clang --samples=$(pwd)/../samples
> $ make fate-rsync
> $ make -j$(nproc)
> $ make fate-dpcm-interplay
>
> The breakage happens in the libavformat/ipmovie.c file. (I also saw a couple other broken tests, so I think there are other source files affected too, but I didn't bisect and pinpoint those failures.)
>
> The issue can be observed with https://martin.st/temp/ipmovie-preproc.c, with `clang -target aarch64-linux-gnu -O3 -o - ipmovie-preproc.c`. The generated code contains differences like this:
>
> --- old.s 2022-01-18 10:30:24.726016244 +0200
> +++ new.s 2022-01-18 10:30:01.650536299 +0200
> @@ -506,7 +506,7 @@
> mov w1, #56
> bl av_log
> add x9, x19, #1104
> - mov w21, #65535
> + mov w21, #-1
> .LBB3_9: // %while.end
> ldr x0, [x19]
> ldr w8, [x0, #44]
Hi @mstorsjo, thanks for the info. I'll revert the patch again for now. It's strange because ANY_EXTEND should really mean "any", i.e. you don't care if it's sign-extend or zero-extend! I suspect there is a bug in codegen somewhere that either relies upon ANY_EXTEND actually being ZERO_EXTEND, or relies upon ANY_EXTEND being consistently the same. I'm worried because the `isSExtCheaperThanZExt` interface definitely allows for the possibility of sometimes choosing one over the other depending upon the types.
Repository:
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CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D114357/new/
https://reviews.llvm.org/D114357
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