bors 9f85cd6f2a Auto merge of #87794 - bonega:enum_niche_prefer_zero, r=nagisa ... Enum should prefer discriminant zero for niche Given an enum with unassigned zero-discriminant, rust should prefer it for niche selection. Zero as discriminant for `Option<Enum>` makes it possible for LLVM to optimize resulting asm. - Eliminate branch when expected value coincides. - Use smaller instruction `test eax, eax` instead of `cmp eax, ?` - Possible interaction with zeroed memory? Example: ```rust pub enum Size { One = 1, Two = 2, Three = 3, } pub fn handle(x: Option<Size>) -> u8 { match x { None => {0} Some(size) => {size as u8} } } ``` In this case discriminant zero is available as a niche. Above example on nightly: ```asm mov eax, edi cmp al, 4 jne .LBB0_2 xor eax, eax .LBB0_2: ret ``` PR: ```asm mov eax, edi ret ``` I created this PR because I had a performance regression when I tried to use an enum to represent legal grapheme byte-length for utf8. Using an enum instead of `NonZeroU8` [here](d683304f5d/src/internal/decoder_incomplete.rs (L90)) resulted in a performance regression of about 5%. I consider this to be a somewhat realistic benchmark. Thanks to `@ogoffart` for pointing me in the right direction! Edit: Updated description		2021-09-13 22:14:57 +00:00
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abi	enum niche allocation grows toward zero if possible	2021-09-13 21:55:14 +02:00
asm	Rollup merge of #88350 - programmerjake:add-ppc-cr-xer-clobbers, r=Amanieu	2021-09-01 09:23:26 +02:00
spec	Auto merge of #88529 - Meziu:master, r=nagisa	2021-09-13 05:48:03 +00:00
lib.rs	use undef for uninitialized bytes in constants	2021-08-25 17:49:28 -04:00
tests.rs	Emit warnings for unused fields in custom targets.	2021-06-17 21:48:02 +02:00