Agner's CPU blog

Posted: **2023-08-25, 3:13:11**

I recently used the testp tool to measure two instructions in unrolled code 1000 times.

rept 1000
mov (r|e)ax, 123
endm

I got the following results:

Code: Select all

mov rax,123        mov eax,123
Clock  Core cyc    Clock  Core cyc
546    442         382    312
558    469         386    312
676    550         388    311

Looking at the results I assumed that it is desirable to use 32-bit rather than 64-bit registers when possible in a 64-bit environment.

Where is my mistake in assuming such a fact?
Can I conclude that both instructions execute the same number of cycles?

(Linux - Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz).

Posted: **2023-08-25, 6:22:49**

An optimizing assembler should code mov rax,123 as mov eax,123 because the result is zero-extended into rax anyway. The two instructions should give identical results. Test results may vary for random reasons. Zero extension cannot be used with negative constants. mov rax,-123 is two bytes longer than mov eax,-123.

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