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.
Index: gcse.c
===================================================================
--- gcse.c (revisione 193848)
+++ gcse.c (copia locale)
@@ -1658,7 +1658,11 @@ compute_transp (const_rtx x, int indx, sbitmap *bm
{
bitmap_iterator bi;
unsigned bb_index;
+ rtx x_addr;
+ x_addr = get_addr (XEXP (x, 0));
+ x_addr = canon_rtx (x_addr);
+
/* First handle all the blocks with calls. We don't need to
do any list walking for them. */
EXECUTE_IF_SET_IN_BITMAP (blocks_with_calls, 0, bb_index, bi)
@@ -1683,7 +1687,7 @@
rtx dest_addr = pair->dest_addr;
if (canon_true_dependence (dest, GET_MODE (dest),
- dest_addr, x, NULL_RTX))
+ dest_addr, x, x_addr))
RESET_BIT (bmap[bb_index], indx);
}
}
