On Tue, May 17, 2016 at 04:17:58AM -0500, Segher Boessenkool wrote: > On Tue, May 17, 2016 at 11:08:53AM +0200, Eric Botcazou wrote: > > > How would it? The shrink-wrapping algorithms do not much care how you > > > write your control flow. The only things I can think of are drastic > > > things like removing some dead code, or converting a switch to a direct > > > jump, but those had better be done for the immediately preceding passes > > > already (register allocation). > > > > But the compiler didn't wait until after shrink-wrapping to emit multiple > > epilogues and can still do that w/o shrink-wrapping. > > It will only ever generate a single epilogue (unless you also count > sibcall epilogues), and that is done after shrink-wrapping. Or you mean > something else and I just don't see it. > > > > I can put back a cleanup_cfg (0) in front if that seems less tricky > > > (or just safer)? > > > > I think you need to evaluate the effects of the change on a set of sources. > > Yeah I'll do that, thanks for the idea.
I built cross-compilers for 30 targets, and built Linux with that. 6 of those failed for unrelated reasons. Of the 24 that do build, five show a few insns difference between having a cleanup_cfg before shrink-wrapping or not (CLEANUP_EXPENSIVE made no difference there). These targets are s390, blackfin, m68k, mn10300, nios2. It turns out that prepare_shrink_wrap *does* care about block structure: namely, it only moves insns from the "head" block to a successor. It then makes a difference when the cleanup_cfg can merge two successor blocks (say, the first is a forwarder block). This happens quite seldomly. So, shall I put a cleanup_cfg back before shrink-wrapping? [ I'm now also looking at what patch #3 (and #2) change; also small stuff ]. Segher
