[PATCH] hppa: Improve expansion of ashldi3 when !TARGET_64BIT

Jeff Law law@redhat.com
Wed Aug 26 21:30:25 GMT 2020

On Wed, 2020-08-26 at 22:23 +0100, Roger Sayle wrote:
> The failure of slsr-13.c is not caused by the patchh3.txt, but the previous patchh2.txt
> that's now on mainline and the gcc-10 branch.  That change provided more accurate
> rtx_costs for hppa, and solved the performance problems with synth_mult.
> These more accurate target rtx_costs are used by the gimple-ssa-strength-reduction.c
> (via a call to mult_by_coeff_cost) to decide whether applying strength reduction would
> be profitable.  This test case, slsr-13.c, assumes that two multiplications by four are
> cheaper than two multiplications by five.   (I believe) This is not the case on hppa which
> has a sh2add instruction, that performs a multiplication by five in one cycle, or exactly
> the same cost as performing a left shift by two (i.e. a multiplication by four).  Oddly, I
> also believe this isn't the case on x86_64, where the similar lea instruction is (sometimes)
> as efficient as left shift by two bits.
Yea, you can do a multiplication by 5 cheap on the PA.  While the x86 can too, I
don't think it's as clear cut a win as the PA, so they may not cost the same as a
multiply by 4 or left shift by 2.

> I suspect that slsr-13.c should be expected to fail on some platforms depending upon 
> a targets instruction set/timings.
Sounds like you're right since it depends on mult_by_coeff_cost under the hood :(
 I presume you or John will xfail it for the PA.

> Unfortunately, to complicate things in our case, it appears that after RTL optimizations,
> performing this strength reduction actually does results in fewer instructions on the PA,
> so it's the right thing to do.  I'll need to study the logic in gimple-ssa-strength to see
> how mult_by_coeff cost is being used; cost(x*4) == cost(x*5), but cost(x*4+y) < cost(x*5+y).
Yea, x*4+y is cheaper than x*5+y on the PA.  The first is a single sh2add, the
second requires an additional add instruction.


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