[LLVMbugs] [Bug 19258] New: Vector extract placement causes 37% regression on fp-convert.

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Wed Mar 26 14:20:29 PDT 2014


            Bug ID: 19258
           Summary: Vector extract placement causes 37% regression on
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: atrick at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This LNT tester:

Showed a 37% regression on fp-convert after this checkin:

Author: atrick
Date: Mon Mar 24 21:18:47 2014
New Revision: 204699

URL: http://llvm.org/viewvc/llvm-project?rev=204699&view=rev
SLP vectorizer: Don't hoist vector extracts of phis.

Extracts coming from phis were being hoisted, while all others were
sunk to their uses. This was inconsistent and didn't seem to serve a
purpose. Changing all extracts to be sunk to uses is a prerequisite
for adding block frequency to the SLP vectorizer's cost model.

My own benchmarking on darwin/x86_64/sandybridge showed a slight improvement
after this checkin: (2.6 vs 2.8s).

If someone can reproduce the regression and show what caused it, we should
definitely teach the vectorizer to avoid the problem.

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