On 2020-03-19 00:56, Segher Boessenkool wrote: > On Tue, Mar 17, 2020 at 03:32:34PM +0000, Michael Matz wrote: >> On Mon, 16 Mar 2020, Richard Sandiford wrote: >>> Similarly for non-call exceptions on other statements. It sounds like >>> what you're describing requires the corresponding definition to happen >>> for memory outputs regardless of whether the asm throws or not, so that >>> the memory appears to change on both excecution paths. Otherwise, the >>> compiler would be able to assume that the memory operand still has its >>> original value in the exception handler. >> >> Well, it's both: on the exception path the compiler has to assume that the >> the value wasn't changed (so that former defines are regarded as dead) or >> that it already has changed (so that the effects the throwing >> "instruction" had on the result (if any) aren't lost). The easiest for >> this is to regard the result place as also being an input. >> >> (If broadened to all instructions under -fnon-call-exceptions, and not >> just to asms will have quite a bad effect on optimization capabilities, >> but I believe with enough force it's already possible now to construct >> miscompiling testcases with the right mixtures of return types and ABIs) > > It's a tradeoff: do we want this to work for almost no one and get PRs > that we cannot solve, or do we generate slightly worse assembler code > for -fnon-call-exceptions? I don't think this is a difficult decision > to make, considering that you already get pretty bad performance with > that flag (if indeed it works correctly at all).
I just realized that changing all outputs to in+out would generate worse code for *every* single asm that has any outputs. Whereas changing outputs to a temporary will not generate any extra code if there is no local try/catch block. I think that is something that should be considered. Then again, if the consensus is that outputs should become in+out, I will implement that. Then there is the issue of those debug stmts being inserted after asms. I can either patch the code that inserts these (in tree-into-ssa.c) as I did earlier, or change the verify routines to ignore debug stmts and the end of a basic block. Which would be preferable?
