I have following code:
sub rsp, 40 ; 00000028H
lea rcx, OFFSET FLAT:$SG4237
call printf
xor eax, eax
add rsp, 40 ; 00000028H
ret 0
Why there is xor eax, eax , instead of xor rax, rax?
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1Because the return value is likely "int", which is 32-bit in most compiler models (LP64/LLP64).– newgreCommented Feb 9, 2017 at 18:28
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Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?– phuclvCommented Jul 22, 2018 at 16:24
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1 Answer
In x64, any operation on a 32-bit register clears the top 32 bits of the corresponding 64-bit register too, so there's no need to use xor rax, rax which would necessitate an extra REX byte for encoding.
answered Feb 9, 2017 at 18:53
