JIT assembly optimizer leaves some redundant branches

[markshannon]

For AArch64 emit__GUARD_IS_TRUE_POP produces the following assembly code:

...
14: 54000041 b.ne 0x1c <_JIT_ENTRY+0x1c>
18: 14000002 b 0x20 <_JIT_ENTRY+0x20>
1c: 14000000 b 0x1c <_JIT_ENTRY+0x1c>
000000000000001c: R_AARCH64_JUMP26 _JIT_JUMP_TARGET

but it should, ideally, emit this:

14: 54000041 b.ne 0x14 <_JIT_ENTRY+0x14>
000000000000001c: R_AARCH64_CONDBR19 _JIT_JUMP_TARGET

This doesn't seem to be an issue for x86, possibly due to the way Clang is setup for AArch64, possibly just an artefact of llvm's code gen.
Either way, rather than rely on llvm eliminating these jumps we can do it in the JIT builder's assembly optimizer.
Currently we don't perform jump fusion, but it would be easy enough to do so.

Linked PRs

• GH-139757: JIT: Remove redundant branches to jumps in the assembly optimizer #140800