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Re: [PATCH], PR target/81550, Rewrite PowerPC loop_align test so it still tests the original target hook


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?


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