[PATCH] vect: Try smaller vector size when SLP split fails

Andrew Stubbs ams@codesourcery.com
Wed Aug 5 13:29:29 GMT 2020

This patch improves SLP performance in combination with some patches I 
have in development to add multiple vector sizes to amdgcn.

The problem is that amdgcn's preferred vector size has 64 lanes, and SLP 
does not support lane masking.  My patches will add smaller vector sizes 
(32, 16, 8, 4, 2) which make the lane masking implicit, but still SLP 
doesn't use them; it simply rejects the first size it sees and gives up.

This patch detects the rejection early and looks to see if there is a 
smaller, more suitable vector size.  The result is many more successful 
SLP testcases.

OK to commit? (I have an x86_64 bootstrap and test in progress.)

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