On Dec 15, 2007, at 1:23 AM, Christopher Lamb wrote:
On Dec 14, 2007, at 10:40 AM, Bill Wendling wrote:
My understanding of a side effect is when an instruction has
un-modeled effects. For instance, "emms" on X86 has side effects that
aren't modeled in the instruction pattern -- it resets the FP
registers after MMX use. Other instructions have no un-modeled side
effects -- e.g., "xor" on X86 or "li" on PPC. Others may have
un-modeled side effects in some situations, but not in others --
e.g.,
loads, in general, have side effects (accessing memory), but loads
from a constant pool don't.
This is the working definition, anyway. In the beginning, it'll be
closely tied to what the "isRematerializable" flag means. I don't
know
what GCC considers side effects. Could you give a summary? It
would be
good to contrast/compare with it.
In processor design discussions all load instructions were termed
"side affecting", as they could cause a page fault. Perhaps the key
here is whether the un-modeled effects of the instruction affect
correct program behavior?
That is the key, yes. :-) We'll leave it up to the individual back
ends to tell us if moving instructions marked as "may having side
effects" affects correct program behavior.
-bw
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