Sequential checked_add and saturating_add of constants don't get combined together

[scottmcm]

pub fn test_saturating(a: u32) -> u32 {
    a.saturating_add(7).saturating_add(13)
}

gets compiled to

playground::test_saturating:
	add	edi, 7
	mov	eax, -1
	cmovb	edi, eax
	add	edi, 13
	cmovae	eax, edi
	ret

but ideally it would get compiled to a.saturating_add(20), aka

playground::test_saturating:
	add	edi, 20
	mov	eax, -1
	cmovae	eax, edi
	ret

Ditto for checked_add.

Playground repro: https://play.rust-lang.org/?gist=eeea31ea85491e38bdd6d456b9f6e47f&version=nightly&mode=release&edition=2015

Found looking at step_by, which would like .next(); .nth(step) to collapse nicely for ranges (#52065).

[hellow554]

I ask myself if this should be fixed in LLVM directly, rather than rust, so two consecutive saturatings/checked adds should be merged?

[scottmcm]

@hellow554 Yes, it'll probably need LLVM changes unless there's some way we could emit it differently that would work better -- this would be rather complicated to do in MIR, AFAICT.

Edit: oops, palm-clicked trying to reply on my phone...

[scottmcm]

Confirmed that LLVM can't do this kind of fold in C++ either. Filed an LLVM bug:

• https://bugs.llvm.org/show_bug.cgi?id=38146 (simpler case of add nsw&sadd.with.overflow)

[scottmcm]

Might get fixed for saturating, or at least easier to fix, if #55286 happens.