Crash on macOS arm64

[joeyparrish]

macOS arm64 builds of FFmpeg are currently crashing on GitHub VMs when testing Shaka Streamer.

[joeyparrish]

FFmpeg gives no indication of what it is doing when it crashes, even with -loglevel trace. Not sure if it's an encoder or decoder issue, or something else. My gut says videotoolbox is to blame.

Letting Rosetta handle the x64 binaries on arm64 seems to work seamlessly.

There are no debug symbols in the builds we shipped, so lldb tells me nothing.

I'm debugging the macOS arm64 builds on my fork.

I will try the binaries directly on the machine that built them to see if there is some difference between the GitHub VM and my actual M1 device.

Then I will try a debug build and lldb to see if we can get a meaningful backtrace.

Finally, if nothing else works, I'll experiment with removing --enable-videotoolbox (not a solution, but may provide a clue) and with adding -fno-stack-check, which is used explicitly in the ffmpeg builds at https://www.osxexperts.net/ .

[joeyparrish]

Builds made on the GitHub VM crash on the system that built them.

A debug build (-g -O0) also crashes.

lldb shows this when it crashes:

* thread #5, name = 'av:h264:df3', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x0000000100498454 ffmpeg`ff_h264_idct_add16intra_neon + 40
ffmpeg`ff_h264_idct_add16intra_neon:
->  0x100498454 <+40>: ldrb   w3, [x7], #0x1
    0x100498458 <+44>: ldrsw  x0, [x5], #0x4
    0x10049845c <+48>: ldrb   w3, [x4, w3, uxtw]
    0x100498460 <+52>: add    x0, x0, x6
Target 0: (ffmpeg) stopped.

Configuring with --disable-neon failed to build.

[joeyparrish]

The M1 mac in the lab couldn't build ffmpeg at all with these instructions. The error was about "illegal text relocations" in some assembly functions for aarch64. I'm not sure why this is different from the build environment of the GitHub VMs.

I couldn't quite solve this, until I found a reference to --enable-pic being required for iOS builds. Since position independent code (PIC) is related to relocation (that's my very surface level understanding), I tried configuring with --enable-pic, and that got the lab machine to build ffmpeg. The resulting build did not crash on the same machine.

I'm trying --enable-pic now on the GitHub VMs to see if this helps fix the crash.