[Bug tree-optimization/53295] New: Vectorizer support for non-constant strided loads depends on gather support overwriting the data-ref with bogus data

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53295

             Bug #: 53295
           Summary: Vectorizer support for non-constant strided loads
                    depends on gather support overwriting the data-ref
                    with bogus data
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: rgue...@gcc.gnu.org
                CC: m...@gcc.gnu.org


If you look at the testcase for PR53185 you can see that we compute the
data-reference for the strided load twice:

Creating dr for *D.1732_13
analyze_innermost: failed: evolution of base is not affine.
        base_address:
        offset from base address:
        constant offset from base address:
        step:
        aligned to:
        base_object: *pretmp.15_35
        Access function 0: {0B, +, (long unsigned int) pretmp.17_38 * 4}_2

this is the failing one (because analyze_innermost does not allow for
non-constant strides - sth to be really fixed).

Creating dr for *D.1732_13
analyze_innermost: success.
        base_address: pretmp.15_35 + (long unsigned int) D.1729_10 * 4
        offset from base address: 0
        constant offset from base address: 0
        step: 0
        aligned to: 128
        base_object: *D.1732_13

that's the one by gather support, guarded by
targetm.vectorize.builtin_gather != NULL (which is only !NULL on x86).  But
this DR is for basic-block analysis, thus does not properly reflect
what the non-constant strided load code would want to use (it doesn't use
that info though).

Possibly the non-constant strided load testcases fail on targets other than
x86 (with support for vectorizing doubles).