alt Hacker NewsUse of the "h" register slices (bits 8..15) by compilers is thankfully pretty rare -- otherwise this would have been noticed much sooner!
Agner Fog's optimization guide says "Any use of the high 8-bit registers AH, BH, CH, DH should be avoided because it can cause false dependences and less efficient code."
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Use of the "h" register slices (bits 8..15) by compilers is thankfully pretty rare
That's unfortunate, because it's precisely why things like this will keep happening.
Agner Fog's optimization guide says "Any use of the high 8-bit registers AH, BH, CH, DH should be avoided because it can cause false dependences and less efficient code."
The sad vicious cycle of compilers not exercising the hardware, and then the hardware designers not paying attention. Using the high 8-bit registers and "implicitly merging" them is one of the ways to reduce the number of instructions and thus improve size optimisation.
> Use of the "h" register slices (bits 8..15) by compilers is thankfully pretty rare -- otherwise this would have been noticed much sooner!
It's actually pretty easy to get compilers to use those, you mainly need a bunch of narrow accesses to neighboring memory. The oodle post contains a godbolt link to pretty ordinary c code triggering this.
I'd guess that you also need some other conditions (multiple in flight stores, high boost speeds) to trigger this.