[Bug tree-optimization/78343] New: Loop is not eliminated

[krister.walfridsson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78343

            Bug ID: 78343
           Summary: Loop is not eliminated
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krister.walfridsson at gmail dot com
  Target Milestone: ---

GCC 6 and trunk generates inefficient code for the loop

  unsigned int test(unsigned int quant)
  {
    unsigned int sum = 0;
    for (unsigned int i = 0; i < quant; ++i){
      sum += quant;
    }

    return sum;
  }

as noted in the tweet
https://twitter.com/lefticus/status/797593368037642244?s=09.

This is a regression introduced in r233207; GCC used to generate

  test:
        movl    %edi, %eax
        imull   %edi, %eax
        ret

before r233207, and it generates a meaningless loop

  test:
        testl   %edi, %edi
        je      .L4
        xorl    %edx, %edx
  .L3:
        addl    $1, %edx
        cmpl    %edx, %edi
        jne     .L3
        movl    %edi, %eax
        imull   %edi, %eax
        ret
  .L4:
        xorl    %eax, %eax
        ret

after that change.