On Tue, Nov 27, 2012 at 8:00 AM, Paolo Bonzini <bonz...@gnu.org> wrote: > Hi, > > this bug triggers in the compilation of QEMU with GCC 4.7.2. It is > latent on trunk because reg_known_value is completely broken. I'll > send a separate patch, but this one applies there too. > > The problem arises when you have -fPIE (or -fPIC) and a huge function > with a lot of references to global variables. Canonicalization of > position-independent addresses is then done over and over for the > same addresses, resulting in quadratic time and memory complexity for > GCSE's compute_transp; hundreds of megabytes of memory are allocated > in plus_constant, > > The fix is to canonicalize the addresses outside the loop, similar to > what is done by the RTL DSE pass. > > gcc 4.4.6: > PRE : 3.83 (24%) usr 0.15 (17%) sys 3.99 (24%) wall 267307 kB (33%) > ggc > > gcc 4.7.2: > PRE : 7.95 (41%) usr 0.40 (40%) sys 8.31 (41%) wall 821017 kB (80%) > ggc > > gcc 4.8.0: > PRE : 6.94 (26%) usr 0.02 ( 4%) sys 6.98 (26%) wall 731 kB ( 0%) > ggc > > gcc 4.7.2 + patch: > PRE : 5.90 (34%) usr 0.02 ( 3%) sys 6.41 (35%) wall 1670 kB ( 1%) > ggc > > Note that the bug is present on older branches too, but it became much > worse sometime between 4.4 and 4.7. > > Bootstrap finished on x86_64-pc-linux-gnu, regtest in progress; ok for > 4.7 and trunk if it passes? > > Paolo > > 2012-11-26 Paolo Bonzini <pbonz...@redhat.com> > > PR rtl-optimization/55489 > * gcse.c (compute_transp): Precompute a canonical version > of XEXP (x, 0), and pass it to canon_true_dependence. >
This caused: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55597 -- H.J.
