On 2020-03-12 16:32, Segher Boessenkool wrote: > On Thu, Mar 12, 2020 at 02:08:18PM +0100, Richard Biener wrote: >>> It wasn't clear from my message above, but: I was mostly worried about >>> requiring the asm to treat memory operands in a certain way when the >>> exception is thrown. IMO it would be better to say that the values of >>> memory operands are undefined on the exception edge. >> >> Rather unspecified. So IMHO on the exception edge the asm() should >> still appear as a def for all outputs so the compiler cannot derive any >> actual values for them. That of course also means that they must not >> appear to be must-defs since we may not DSE earlier stores for example. > > So make all outputs of an asm that may throw (w/ -fnon-call-exceptions) > inout operands instead? That works for registers exactly the same, too?
Should gcc do this or would it be the user's responsibility? For memory operands I don't think anything changes if you replace "=m" by "+m". However changing each "=r" to "+r" would certainly generate less optimal code. >> If we manage to get the unspecified values correct in SSA then we don't >> need to say whether the asm may clobber them before or after throwing. > > Yeah. Which users will *never* get right, that is, it would be hard to > use any such interface correctly. > > > Segher >
