On Wed, 20 Jan 2016, Jakub Jelinek wrote: > On Wed, Jan 20, 2016 at 10:24:40AM +0100, Richard Biener wrote: > > > We could diagnose a statement expression in "m", but not sure if that is > > > all > > > that can get wrong, or if all statement expressions are problematic. > > > > I thought about either requiring an lvalue here or at least diagnosing > > that a non-lvalue might end up using a memory temporary. > > Requiring an lvalue sounds wrong, "m" input can be validly e.g. const object > that > can't be assigned to.
Ok, so maybe the term "lvalue" is wrong but certainly an arbitrary rvalue is not "valid". > Furthermore, I'm really afraid changing that would break > too much stuff. > We had until GCC 4.7 a warning: > warning (0, "use of memory input without lvalue in " > "asm operand %d is deprecated", i + noutputs); > but 1) it wasn't anywhere near frontend, it was during expansion > 2) it wasn't really checking lvalue, but whether the operand is a MEM or not > > > > > > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? > > > > > > > > What happens if we just do _not_ mark the memory input addressable? > > > > Shouldn't IRA/LRA in the end satisfy the constraint by spilling > > > > a non-memory input and using the spill slot? > > > > > > Well, if you want to make broken testcases work, it is always possible > > > to call say prepare_gimple_addressable, but I'd think it is preferrable > > > to tell people that what they do is really going to do something different > > > from what they expect (that the operand, while being a memory input, will > > > be some temporary containing a copy of the value rather than than the > > > variable itself. > > > > Sure, I'm just thinking that diagnosing sth at gimplification time > > feels wrong ... after all we can make it unexpected but valid GIMPLE. > > We already do diagnose tons of other cases for inline asm at > gimplification time. I can replace the error with a warning followed by > copying it to addressable memory, that seems to be what the older gccs were > doing during expansion after issuing above mentioned warning. That sounds better then though it would be nice to warn from the FE somehow. Richard.
