On Mon, Jun 16, 2014 at 12:57 PM, Bernd Schmidt <ber...@codesourcery.com> wrote: > There's code in regimplification that makes us use an extra temporary > when we encounter a call returning a non-BLKmode structure. This seems > somewhat inefficient and unnecessary, and when used from the > lower-addr-spaces pass I'm working on it leads to problems further > down that look like tree-ssa bugs that I wasn't able to clearly > disentangle. > > Here's what happens on compile/pr51761.c. Regimplification has the > following effect, creating an extra temporary _6: > > - D.1378 = fooD.1373 (aD.1377); > + _6 = fooD.1373 (aD.1377); > + # .MEMD.1382 = VDEF <.MEMD.1382> > + D.1378 = _6; > > SRA turns this into: > > _6 = fooD.1373 (aD.1377); > # VUSE <.MEM_3> > SR$2_7 = MEM[(struct S *)&_6];
clearly bogus - _6 is a register, you can't use a MEM on it. > Somehow, the address of &_6 doesn't count as a use, and the DCE pass decides > it is unused: > > Eliminating unnecessary statements: > Deleting LHS of call: _6 = foo (a); > > However, the statement > SR$2_7 = MEM[(struct S *)&_6]; > is still present, and we have an SSA name without a definition, leading to a > crash. > > Rather than figure all this out, I decided to try making the > regimplification not generate the extra copy in the first place. The > testsuite seems to agree with me that it's unnecessary. Bootstrapped and > tested on x86_64-linux, ok? Ok. The code looks bogus anyway in that it generates a SSA name for sth not is_gimple_reg_type (). Thanks, Richard. > > Bernd
