Use 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."
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."