Improve IR for calls with immediate return values #7645
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There's no need to allocate a return slot for anykind of immediate return value, not just not for nils. Also, when the return value is ignored, we only have to copy it to a temporary alloca if it's actually required to call drop_ty on it.
We currently still handle immediate return values a lot like non-immediate ones. We provide a slot for them and store them into memory, often just to immediately load them again. To improve this situation, trans_call_inner has to return a Result which contains the immediate return value. Also, it also needs to accept "No destination" in addition to just SaveIn and Ignore. Since "No destination" isn't something that fits well into the Dest type, I've chosen to simply use Option<Dest> instead, paired with an assertion that checks that "None" is only allowed for immediate return values.
bors
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There are lots of unneeded allocas and Store/Load cycles for calls with immediate return values. This is a first step towards removing that, allowing immediate return values to be directly returned from trans_call_inner and trans_lang_call (for now), instead of always stuffing them into an alloca. For now, only a few things take advantage of the new behaviour, but this already saves 16k allocas and 43k lines in total in the unoptimized IR for librustc. Running "make check" under time shows that CPU time for the unoptimized test suite is reduced by about 7%.
flip1995
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Sep 28, 2021
New lint `if_then_panic` changelog: add the new lint [`if_then_panic`] fix rust-lang#7645
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There are lots of unneeded allocas and Store/Load cycles for calls with
immediate return values. This is a first step towards removing that, allowing
immediate return values to be directly returned from trans_call_inner and
trans_lang_call (for now), instead of always stuffing them into an alloca.
For now, only a few things take advantage of the new behaviour, but this
already saves 16k allocas and 43k lines in total in the unoptimized IR
for librustc. Running "make check" under time shows that CPU time for
the unoptimized test suite is reduced by about 7%.