[PATCH] D114357: [CodeGen][AArch64] Ensure isSExtCheaperThanZExt returns true for negative constants

Tue Jan 18 00:32:46 PST 2022

mstorsjo added a comment.

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]

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D114357/new/

https://reviews.llvm.org/D114357