[llvm-bugs] [Bug 28987] New: Regression in removing pointer range loops initializing dead variables.

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28987

            Bug ID: 28987
           Summary: Regression in removing pointer range loops
                    initializing dead variables.
           Product: new-bugs
           Version: 3.9
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: edy.burt at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following function is optimized out in clang 3.8, but not 3.9:

void test() {
  double a[4];
  for(double *p = a, *end = a + 4; p < end; p++)
    *p = 0.0;
}

The optimized IR on 3.9 is more or less:

define void @test() {
entry-block:
  %a = alloca [4 x double], align 8
  %a6 = ptrtoint [4 x double]* %a to i64
  %0 = bitcast [4 x double]* %a to i8*
  call void @llvm.lifetime.start(i64 32, i8* %0)
  %scevgep = getelementptr [4 x double], [4 x double]* %a, i64 1, i64 0
  %scevgep1 = ptrtoint double* %scevgep to i64
  %scevgep2 = getelementptr [4 x double], [4 x double]* %a, i64 0, i64 1
  %scevgep23 = ptrtoint double* %scevgep2 to i64
  %1 = icmp ugt i64 %scevgep1, %scevgep23
  %umax = select i1 %1, i64 %scevgep1, i64 %scevgep23
  %umax45 = inttoptr i64 %umax to i8*
  %2 = xor i64 %a6, -1
  %uglygep = getelementptr i8, i8* %umax45, i64 %2
  %uglygep7 = ptrtoint i8* %uglygep to i64
  %3 = add i64 %uglygep7, 8
  %4 = and i64 %3, -8
  call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 %4, i32 8, i1 false)
  call void @llvm.lifetime.end(i64 32, i8* %0)
  ret void
}

If the llvm.lifetime.{start,end} calls are removed, running opt again will
reduce this to just "ret void".

Any other uses of %a will also prevent the memset and everything before it to
be removed, *even* if the entire array is statically overwritten.

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