Why did GCC generate mov %eax,%eax and what does it mean?

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In x86-64, 32-bit instructions implicitly zero-extend: bits 32-63 are cleared (to avoid false dependencies). So sometimes that’s why you’ll see odd-looking instructions. (Is mov %esi, %esi a no-op or not on x86-64?)

However, in this case the previous mov-load is also 32-bit so the high half of %rax is already cleared. The mov %eax, %eax appears to be redundant, apparently just a GCC missed optimization.

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