[llvm-bugs] [Bug 33642] New: [InlineCost] CallAnalyzer::visitGetElementPtr can return true for GEPS which are not cost-free

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33642

            Bug ID: 33642
           Summary: [InlineCost] CallAnalyzer::visitGetElementPtr can
                    return true for GEPS which are not cost-free
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: eastig at yandex.ru
                CC: llvm-bugs at lists.llvm.org

In the following IR produced from the code of a simple memcopy function GEPs
are not free:

while.cond:                                       ; preds = %while.body, %entry
  %dest.addr.0 = phi i8* [ %dest, %entry ], [ %incdec.ptr1, %while.body ]
  %src.addr.0 = phi i8* [ %src, %entry ], [ %incdec.ptr, %while.body ]
  %tobool = icmp eq i32 %size.addr.0, 0
  br i1 %tobool, label %while.end, label %while.body

while.body:                                       ; preds = %while.cond
  %dec = add nsw i32 %size.addr.0, -1
  %incdec.ptr = getelementptr inbounds i8, i8* %src.addr.0, i32 1
  %0 = load i8, i8* %src.addr.0, align 1, !tbaa !12
  %incdec.ptr1 = getelementptr inbounds i8, i8* %dest.addr.0, i32 1
  store i8 %0, i8* %dest.addr.0, align 1, !tbaa !12
  br label %while.cond

while.end:                                        ; preds = %while.cond

For x86 and ARM they are lowered into ADD instructions.

The analysis of GEPs is done in CallAnalyzer::visitGetElementPtr.
There is the code:

  if (IsGEPOffsetConstant(I)) {
    if (SROACandidate)
      SROAArgValues[&I] = SROAArg;

    // Constant GEPs are modeled as free.
    return true;
  }

For all GEPs with constant offsets true is returned. The example above shows
when GEPs with constant offsets are not free. So we need to call isGEPFree to
check if GEP is actually free of cost.

Note: there is a patch (https://reviews.llvm.org/D33685) of isGEPFree to use
TTI::getUserCost instead of TTI::getGEPCost. TTI::getUserCost will catch the
cases above.

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