Bug 56595

Summary: Tree-ssa-pre can create loop carried dependencies which prevent loop vectorization.
Product: gcc Reporter: Yuri Rumyantsev <ysrumyan>
Component: tree-optimizationAssignee: Not yet assigned to anyone <unassigned>
Severity: normal CC: anlauf, izamyatin
Priority: P3    
Version: 4.8.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed: 2013-03-11 00:00:00
Bug Depends on:    
Bug Blocks: 53947    
Attachments: testcase

Description Yuri Rumyantsev 2013-03-11 13:34:38 UTC
In some cases pre can create loop carried dependencies spanning multiple iterations aka scalar replacement. This deficiency can be illustrated with attached test-case. After pre for stmt

            DO I = 0,I2
               T1 = 0.5D0 * (U1(I,J,K)    + U1(I+1,J,K))

pre creates loop carried dependence:

  <bb 172>:
  pretmp_690 = MEM[(real(kind=8)[0:] *)pretmp_675][pretmp_689];
  <bb 107>:
  # i_1 = PHI <0(172), i_437(175)>
  # prephitmp_691 = PHI <pretmp_690(172), _440(175)>

Note that in this particular test-case we have arrays with unknown stride1. If we have arrays with stride1 == 1 such transformation does not happen as for the following simple test-case which is successfully vectorized:

	subroutine bar(a,b,c,d,n, m)
	integer n, m
	real*8 a(n,*), b(n,*), c(n,*), d(n,*)
	do j=1,m
	do i=1,m
	x1 = 0.5 * (a(i,j) + a(i+1,j))
	x2 = 0.5 * (b(i,j) + b(i+1,j))
	x3 = 0.5 * (c(i,j) + c(i+1,j))
	d(i,j) = (x1 + x2 + x3) / 3.0
Comment 1 Yuri Rumyantsev 2013-03-11 13:38:25 UTC
Created attachment 29636 [details]

This test must be compiled with the following options for x86:

-ffree-line-length-none -m64 -Ofast -march=core-avx-i -mavx
Comment 2 Richard Biener 2013-03-11 14:25:02 UTC
Confirmed (and known).  See inhibit_phi_insertion in PRE.

I bet you find some duplicate in the list of missed-vectorization bugs.

Loop store-motion can result in similar issues.

Note that an issue with limiting things even further is that PRE is limited
even when the resulting loop is _not_ vectorized.  Which is of course bad.

So the best solution is to teach the vectorizer to handle this kind of
dependency (after all user code can be written in that way from the start).
Comment 3 Richard Biener 2013-09-26 14:57:45 UTC

*** This bug has been marked as a duplicate of bug 35229 ***