Apple platforms: Disabled frame pointer elimination causes perf issues and is not in line with what clang does

[hkratz]

PR #85706 causes massive performance issues for not-inlined leaf functions since the frame pointer and the link register are now always saved and restored at the beginning and end of a function, even if this function e.g. just returns a constant. In contrast to the PR description this is not what clang does on Macos aarch64. The clang default on Macos is "frame-pointer"="non-leaf" instead.

E.g. clang compiles

unsigned long test(unsigned long num) {
    return num % 64;
}

to this LLVM IR (with clang -c test.c -S -emit-llvm -O3)

[...]
; Function Attrs: norecurse nounwind readnone ssp uwtable
define i64 @test(i64 %0) local_unnamed_addr #0 {
  %2 = and i64 %0, 63
  ret i64 %2
}

attributes #0 = { norecurse nounwind readnone ssp uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="non-leaf" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="true" "probe-stack"="__chkstk_darwin" "stack-protector-buffer-size"="8" "target-cpu"="apple-a12" "target-features"="+aes,+crc,+crypto,+fp-armv8,+fullfp16,+lse,+neon,+ras,+rcpc,+rdm,+sha2,+v8.3a,+zcm,+zcz" "unsafe-fp-math"="false" "use-soft-float"="false" }
[...]

which leads to this assembly:

_test:                                  ; @test
	.cfi_startproc
; %bb.0:
	and	x0, x0, #0x3f
	ret
	.cfi_endproc

Since the inclusion of the PR Rust Nightly sets "frame-pointer"="all", which causes this assembly to be generated:

_test:                                  ; @test
   .cfi_startproc
; %bb.0:
   stp	x29, x30, [sp, #-16]!           ; 16-byte Folded Spill
   mov	x29, sp
   .cfi_def_cfa w29, 16
   .cfi_offset w30, -8
   .cfi_offset w29, -16
   and	x0, x0, #0x3f
   ldp	x29, x30, [sp], #16             ; 16-byte Folded Reload
   ret
   .cfi_endproc

@jrmuizel: FYI

[jrmuizel]

Indeed. Here's what clang does:
https://github.com/llvm/llvm-project/blob/c3cc14f87f78f8172b74175bbd2557cfb9384900/clang/lib/Driver/ToolChains/Clang.cpp#L604

For AArch64 it omits the frame pointer for leaf functions. Interestingly at https://github.com/llvm/llvm-project/blob/c3cc14f87f78f8172b74175bbd2557cfb9384900/clang/lib/Driver/ToolChains/Clang.cpp#L517 you'd think the same would apply ARM and Thumb on Darwin but with a closer reading it appears that's not the case.

It looks like Rust doesn't currently support non-leaf frame pointer omission so that's going to need to be hooked up before this can fixed.

Can you elaborate on the massive performance issues? Do you have a benchmark where you noticed this?

[inquisitivecrystal]

@rustbot label A-codegen C-bug I-prioritize I-slow O-ios O-macos regression-from-stable-to-nightly T-compiler

Not entirely sure "bug" and "regression" fit a deliberate change, but since it seems to have been based on a false assumption and have caused performance issues I'm applying them. If someone thinks they don't fit, feel free to undo.

[hkratz]

I have to admit that I was writing a non-inlined partial load function using Neon intrinsics which was heavily affected by this. So it might not be as bad in the general case. I will run some benchmarks.

[hkratz]

The time for an idem function call (returning the u64 passed in as an argument) on Apple M1 is:

• 0.34 ns inlined
• 0.96 ns without saving the frame pointer
• 1.44 ns with saving the frame pointer

The performance difference in the real world applications I have measured (ripgrep, rustc) is negligable. The size of generated binaries is somewhat surprising: The __text section size is 1817500 on ripgrep without saving the fp vs 1846828 (x 1.016) saving the fp. However the __unwind_info and __const sections are much larger leading to an overall stripped binary size of 2926720 without saving the fp vs 2811168 saving the fp.

So all in all while I still think that using "frame-pointer"="non-leaf" on Macos is generally a good idea since it has the potential to affect performance in rare cases it is not as urgent as I thought. Mea culpa.