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Re: [PATCH], PR target/81550, Rewrite PowerPC loop_align test so it still tests the original target hook
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Michael Meissner <meissner at linux dot vnet dot ibm dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, David Edelsohn <dje dot gcc at gmail dot com>, Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>
- Date: Wed, 24 Jan 2018 12:35:38 -0600
- Subject: Re: [PATCH], PR target/81550, Rewrite PowerPC loop_align test so it still tests the original target hook
- Authentication-results: sourceware.org; auth=none
- References: <20180124052755.GA8250@ibm-tiger.the-meissners.org>
On Wed, Jan 24, 2018 at 12:27:55AM -0500, Michael Meissner wrote:
> As Segher and I were discussing over private IRC, the root cause of this bug is
> the compiler no long generates the BDNZ instruction for a count down loop,
> instead it decrements the index in a GPR and does a branch/comparison on it.
Yes, ivopts makes a bad decision (it uses stride 8 for all IVs, it should
keep one with stride -1 for the loop counter, for optimal code; it also
does three separate increments for the three memory accesses, which is
a bit excessive here).
> In doing so, it now unrolls the loop twice, and and the resulting loop is too
> big for the target hook TARGET_ASM_LOOP_ALIGN_MAX_SKIP. This means the loop
> isn't aligned to a 32 byte boundary.
It's not really unrolling, it is bb-reorder copying an RTL block. However,
even if you disable it you still get 9 insns on some configurations, so
your patch does not hide the problem :-(
Although, hrm, in your patch you also change "int i" to "long i"; that
alone seems to be enough to fix everything? Could you check that please?