Hi I am a beginner in assembly language, is there a difference between movl (%rdi,%r12), %r10d and movl (%rdi,%r12,4), %r10d? I tried both of them and they both seem to do the same job-- saving the element of %rdi at %r12 position to %r10d. Is the 4 in the 2nd example really necessary?
x86-64 accessing element of array
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Yes, they are different.
Suppose for example that
%rdi = 0x12340000and%r12 = 8. Thenmovl (%rdi,%r12), %r10dadds%r12to%rdito form an effective address, so you load a 32-bit dword from address0x12340008. The4is the scale parameter of the SIB addressing mode; it causes%r12to be multiplied by 4 before adding to%rdi, so withmovl (%rdi,%r12,4), %r10dyou load a 32-bit dword from address0x12340020instead.See the GNU as manual: "If no
scaleis specified,scaleis taken to be 1."If you try it with an array defined like
and
%rdicontains the address offoo, with%r12 = 8as above, thenmovl (%rdi,%r12), %r10dwill load%r10dwith the value5, butmovl (%rdi,%r12,4), %r10dwill load it with the value17. In other words, it is a question of whether you are treating%rdias a count of bytes, or a count of 32-bit ints.If they appear to work the same in your test, then either your test is buggy, or the two different addresses happen by coincidence to contain the same value, or your
%r12is zero, or your assembler is broken (unlikely).