On Tue, 2023-02-28 at 16:02 -0800, H. Peter Anvin wrote: > > > Ewww. Couldn't the assembler choose to omit the REX prefix then? It > > does more tricksy things than that already. > > > > I almost prefer having the prefix but (in the morning) if you > > prefer I can put it back as it was with a comment about the zero- > > extension. > > > > Like it or not, that's how the assembler currently works. Last time someone told me "you can't tell the assembler what you actually mean; you have to tell it something different" I threw my toys out of the pram a little bit and implemented all of the 16-bit '-m16' support in LLVM/clang. On Tue, 2023-02-28 at 19:03 -0500, Brian Gerst wrote: > > commit a7bea8308933aaeea76dad7d42a6e51000417626 > Author: Jan Beulich <JBeulich@xxxxxxxx> > Date: Mon Jul 2 04:31:54 2018 -0600 > > x86/asm/64: Use 32-bit XOR to zero registers > > Some Intel CPUs don't recognize 64-bit XORs as zeroing idioms. Zeroing > idioms don't require execution bandwidth, as they're being taken care > of in the frontend (through register renaming). Use 32-bit XORs instead. > > Not that speed is important here, but it's good to be consistent > across the whole kernel. Someone will eventually come by and fix it > up anyways, as there have been a few of these patches already in the > git history. So there literally is a better encoding of this mnemonic which isn't just a byte shorter, but is also faster to execute. And one *hopes* that the compiler knows about it... yep... so why in $DEITY's name can't I type what I mean into an asm file? Isn't asm intricate enough already? Meh, life's too short. Not shaving that particular yak today. I've put the git tree back to using %edx, with a comment that it zero-extends.
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