| Issue |
146541
|
| Summary |
[BOLT][AArch64] unable to run BOLT on libffi
|
| Labels |
BOLT
|
| Assignees |
|
| Reporter |
geofft
|
We originally ran into this building CPython for python-build-standalone, but here's a minimal reproducer:
```
ubuntu@ubuntu:~/libffi-3.5.1$ ./configure CFLAGS=-Wl,-emit-relocs
ubuntu@ubuntu:~/libffi-3.5.1$ make -j8
ubuntu@ubuntu:~/libffi-3.5.1$ llvm-bolt-21 aarch64-unknown-linux-gnu/.libs/libffi.so.8.2.0 -o=foo
BOLT-INFO: shared object or position-independent executable detected
BOLT-INFO: Target architecture: aarch64
BOLT-INFO: BOLT version: <unknown>
BOLT-INFO: first alloc address is 0x0
BOLT-INFO: creating new program header table at address 0x200000, offset 0x200000
BOLT-INFO: enabling relocation mode
BOLT-WARNING: function ffi_go_closure_SYSV/1 has an object detected in a padding region at address 0x18048
BOLT-WARNING: function __clear_cache/1 has an object detected in a padding region at address 0x18068
BOLT-WARNING: 1 collisions detected while hashing binary objects. Use -v=1 to see the list.
BOLT-INFO: number of removed linker-inserted veneers: 0
BOLT-INFO: 0 out of 134 functions in the binary (0.0%) have non-empty execution profile
BOLT-INFO: removed 17 empty blocks
BOLT-ERROR: cannot relax ADR in non-simple function trampoline_code_table/1
```
This is with https://github.com/libffi/libffi/releases/download/v3.5.1/libffi-3.5.1.tar.gz, Ubuntu Plucky, and the the current version from apt.llvm.org (bolt-21=1:21~++20250630095742+59a7185dd9d6-1~exp1~20250630215911.150).
I think this is because [trampoline_code_table is defined as hand-written assembly](https://github.com/libffi/libffi/blob/v3.5.1/src/aarch64/sysv.S#L580) that does an `adr` and has no `nop`s for BOLT to work with.
A naive attempt at throwing some extra `nop`s into that function gave a worse failure, which I guess makes sense because it's loading some PC-relative addresses and I surely got those addresses wrong. But also it would be cool to be able to run BOLT without requiring changes in third-party code.
FWIW I think I would be happy leaving this un-optimized, if that's an option, but the `adr` relaxation pass looks unconditional and it seems to error out instead of skipping optimizing this one function. I don't totally understand whether this pass really needs to be mandatory.
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