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Re: [4.9] PR 62146

From: Jeff Law <law at redhat dot com>
To: Easwaran Raman <eraman at google dot com>
Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Brooks Moses <bmoses at google dot com>
Date: Wed, 03 Sep 2014 12:36:55 -0600
Subject: Re: [4.9] PR 62146
Authentication-results: sourceware.org; auth=none
References: <CAPK5YPYawDfrFe45fR7s+y20UeXonHAW4jVKQGnKp3OFK71cHw at mail dot gmail dot com> <5400DD5D dot 1070009 at redhat dot com> <CAPK5YPbbK7AezJGccfvuYnp3i5herVBy2Fe3P29h2ftSC7cEzA at mail dot gmail dot com>

On 09/02/14 12:52, Easwaran Raman wrote:

It turns out that the REG_EQUAL note is removed on a hoisted
instruction (relevant code is in dead_or_predicable in ifcvt.c) if the
source of the move instruction is not a function invariant. In this
case, the source is a function invariant (constant) and so that
doesn't kick in. I don't understand why this exemption for function
invariant is there and the original thread in
https://gcc.gnu.org/ml/gcc/2005-05/msg01710.html doesn't explain
either. Should I just remove the REG_EQUAL notes of all hoisted
instructions or are there cases where it is safe to leave the note?

I suspect it was left in in an attempt to keep as many REG_EQUAL notesas possible as the notes can help later optimizations. But even thoseequivalences are not necessarily safe.

I'm pretty sure the right thing to do here is just drop the noteregardless of whether or not its an invariant.


jeff

Follow-Ups:
- Re: [4.9] PR 62146
  - From: Easwaran Raman

References:
- Re: [4.9] PR 62146
  - From: Easwaran Raman

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