On Wed, 14 Feb 2024, Richard Biener wrote:
> For the testcase in PR113910 we spend a lot of time in PTA comparing
> bitmaps for looking up equivalence class members. This points to
> the very weak bitmap_hash function which effectively hashes set
> and a subset of not set bits. The following improves it by mixing
> that weak result with the population count of the bitmap, reducing
> the number of collisions significantly. It's still by no means
> a good hash function.
>
> One major problem with it was that it simply truncated the
> BITMAP_WORD sized intermediate hash to hashval_t which is
> unsigned int, effectively not hashing half of the bits. That solves
> most of the slowness. Mixing in the population count improves
> compile-time by another 30% though.
>
> This reduces the compile-time for the testcase from tens of minutes
> to 30 seconds and PTA time from 99% to 25%. bitmap_equal_p is gone
> from the profile.
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu, will
> push to trunk and branches.
Ha, and it breaks bootstrap because I misunderstood
bitmap_count_bits_in_word (should be word_s_). Fixing this turns
out that hashing the population count doesn't help anything
so I'm re-testing the following simpler variant, giving up on the
cheap last 25% but solving the regression as well.
Richard.
>From a76aebfdc4b6247db6a061e6395fd088a5694122 Mon Sep 17 00:00:00 2001
From: Richard Biener <rguent...@suse.de>
Date: Wed, 14 Feb 2024 12:33:13 +0100
Subject: [PATCH] tree-optimization/113910 - huge compile time during PTA
To: gcc-patches@gcc.gnu.org
For the testcase in PR113910 we spend a lot of time in PTA comparing
bitmaps for looking up equivalence class members. This points to
the very weak bitmap_hash function which effectively hashes set
and a subset of not set bits.
The major problem with it is that it simply truncates the
BITMAP_WORD sized intermediate hash to hashval_t which is
unsigned int, effectively not hashing half of the bits.
This reduces the compile-time for the testcase from tens of minutes
to 42 seconds and PTA time from 99% to 46%.
PR tree-optimization/113910
* bitmap.cc (bitmap_hash): Mix the full element "hash" to
the hashval_t hash.
---
gcc/bitmap.cc | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/gcc/bitmap.cc b/gcc/bitmap.cc
index 6cf326bca5a..459e32c1ad1 100644
--- a/gcc/bitmap.cc
+++ b/gcc/bitmap.cc
@@ -2706,7 +2706,7 @@ bitmap_hash (const_bitmap head)
for (ix = 0; ix != BITMAP_ELEMENT_WORDS; ix++)
hash ^= ptr->bits[ix];
}
- return (hashval_t)hash;
+ return iterative_hash (&hash, sizeof (hash), 0);
}
--
2.35.3
