Messages in this thread | | | Date | Thu, 01 Nov 2007 10:41:19 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt |
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Keir Fraser wrote: > volatile prevents the asm from being 'moved significantly', according to the > gcc manual. I take that to mean that reordering is not allowed. >
That phrase doesn't appear in the gcc manual; in fact, it specifically says that reordering can happen:
The `volatile' keyword indicates that the instruction has important side-effects. GCC will not delete a volatile `asm' if it is reachable. (The instruction can still be deleted if GCC can prove that control-flow will never reach the location of the instruction.) Note that even a volatile `asm' instruction can be moved relative to other code, including across jump instructions. For example, on many targets there is a system register which can be set to control the rounding mode of floating point operations. You might try setting it with a volatile `asm', like this PowerPC example:
asm volatile("mtfsf 255,%0" : : "f" (fpenv)); sum = x + y;
This will not work reliably, as the compiler may move the addition back before the volatile `asm'. To make it work you need to add an artificial dependency to the `asm' referencing a variable in the code you don't want moved, for example:
asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv)); sum = x + y;
I take from this that it is not a good idea to assume "asm volatile" has any ordering effects at all.
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