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[Bug target/82447] [IVOPTS] Consider removing cmp instruction while iterating on an array of known bound
- From: "amker at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 09 Oct 2017 15:02:02 +0000
- Subject: [Bug target/82447] [IVOPTS] Consider removing cmp instruction while iterating on an array of known bound
- Auto-submitted: auto-generated
- References: <bug-82447-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82447
--- Comment #4 from amker at gcc dot gnu.org ---
(In reply to Jakub Jelinek from comment #2)
> Indeed. But when the body is:
> sum += a[size - 1 - i];
> while base 128U with step -1U is considered, it isn't found as the cheapest
> for some reason either.
On 64-bit targets, candidate [128, -1] is added in unsigned int type, while
address IV has pointer type. IVOPTs simply skips the candidate because it has
smaller precision than the IV use.
On 32-bit targets, I increased cost in case of invariant expression on purpose
in order to avoid generating too many invariant expressions. So I think this
strategy works better for large cases but not for the simplest cases. Probably
we should special case simple case with only one address type IV.