Coline Lelong-Pantel wrote:
Hi I've compiled a short program with two different gcc versions (3.2.3 and 3.4.3) but it seems that the produced assembly code isn't the same. In fact 3.4.3 produce a more important code. For example when gcc 3.2.3 produces : movl $0x405c28f6, -4(%ebp) gcc 3.4.3 may use: movl $0x405c28f6, %eax movl %eax, -4(%ebp) I wonder why it now uses %eax register ? And is there any way to prevent this, in order to have similar assembly code with both gcc versions ? Maybe some kind of option ? Thanks for your answers.
Both gcc versions you quote are fairly old; if efficiency interests you, you might try a current version. Were you able to show a significant difference in performance on a CPU which is important to you? I would guess the most important reason for avoiding use of eax here would be to keep that register available for other use. The optimization could be affected by compiler switches (-Os, -O2) or by selection of -march option, and many other possibilities. Newer CPUs have hardware merging of operations, as well as stalls on certain immediate data operations, so it is impossible to know the performance implications from what you have presented.
