Optimization regression in 1.32+

[adrian17]

The following function:

pub fn num_to_digit(num: char) -> u32 {
    if num.is_digit(8) { num.to_digit(8).unwrap() } else { 0 }
}

With 1.31 produced the following:

  add edi, -48
  xor eax, eax
  cmp edi, 8
  cmovb eax, edi
  ret

Which looks reasonable, as unwrap() should never fail.
But in 1.32 and above it produces an unnecessary panic branch:

  push rax
  mov ecx, edi
  and ecx, -8
  xor eax, eax
  cmp ecx, 48
  jne .LBB0_3
  add edi, -48
  mov eax, edi
  cmp edi, 8
  jae .LBB0_2
.LBB0_3:
  pop rcx
  ret
.LBB0_2:
  lea rdi, [rip + .L__unnamed_1]
  call qword ptr [rip + _ZN4core9panicking5panic17h6f50c0de2dcd7974E@GOTPCREL]
  ud2

Interestingly, when I manually inline is_digit:

rust/src/libcore/char/methods.rs

Lines 58 to 61 in 89573b3

	#[inline]
	pub fn is_digit(self, radix: u32) -> bool {
	self.to_digit(radix).is_some()
	}

pub fn num_to_digit_fast1(num: char) -> u32 {
    if num.to_digit(8).is_some() { num.to_digit(8).unwrap() } else { 0 }
}

Or just use unwrap_or:

pub fn num_to_digit_fast2(num: char) -> u32 {
    num.to_digit(8).unwrap_or(0)
}

It produces good code on all compiler versions.

Godbolt instance I tested this on: https://godbolt.org/z/8yg7x0

[andjo403]

have bisected this regression and the PR where the regression started is #55932

seems like the inliner are not able to inline the function any more after that PR

[mati865]

Worth noting with -Zmir-opt-level=2 the regression is gone:

example::num_to_digit_fast:
  add edi, -48
  xor eax, eax
  cmp edi, 8
  cmovb eax, edi
  ret

example::num_to_digit_fast2:
  add edi, -48
  xor eax, eax
  cmp edi, 7
  cmovbe eax, edi
  ret

[bugadani]

As far as I can see, this can be solved if mir inlining gets out of mir-opt-level 2.

[adrian17]

FYI, looks like this got partially fixed somewhere in 1.64 (no clue by which commit though):

example::num_to_digit:
  lea ecx, [rdi - 48]
  mov edx, edi
  and edx, -8
  xor eax, eax
  cmp edx, 48
  cmove eax, ecx
  ret

And 1.65 optimized it more, it looks mostly the same as before the regression now (only difference being add -> lea):

example::num_to_digit:
  lea ecx, [rdi - 48]
  xor eax, eax
  cmp ecx, 8
  cmovb eax, ecx
  ret