gcc inline - operand type mismatch for 'cmp'

Question

I get the following error:

Assembler messages:
Error: operand type mismatch for `cmp'

The only cmp in my code is:

"cmpl %eax, $15\n\t"

I don't get what is wrong with that - I have a register and an immediate value which should be implicitly sign extended to 32bit.

Answer 1

According to Official Intel ISA Reference 3.2 Instructions (A-L)

CMP EAX, imm32

Which means either in Intel Syntax

cmp eax, 15D

or in AT&T Syntax:

cmpl $15, %eax

Just the opposite :)

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cmp doesn't have a form that subtracts the reg/mem from an immediate. Instead, use the opposite branch / cmov condition if you wanted to check something other than equality.