[LLVMbugs] [Bug 16600] New: IndVars should preserve nsw when widening IV

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http://llvm.org/bugs/show_bug.cgi?id=16600

            Bug ID: 16600
           Summary: IndVars should preserve nsw when widening IV
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: atrick at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 10869
  --> http://llvm.org/bugs/attachment.cgi?id=10869&action=edit
Jonas' test case

Passes downstream from IndVars like to see the NSW flags in tact. IV widening
currently removes them because it uses SCEVExpander. I think this is fixable.

Here's an example from Jonas Wagner

I'm using LLVM to reason about memory safety of programs. One goal is to prove
that certain array accesses are always safe.

Currently, one of these proofs fails because of a missing no-signed-wrap (nsw)
flag. I found that it has been "lost" during the SimplifyIndVar pass. Here's
the example:

int foo(int a[]) {
    int sum = 0;
    for (int i = 0; i < 1000; ++i)
        sum += a[i];
    return sum;
}

// *** IR Dump Before Induction Variable Simplification ***
// for.body:                                         ; preds = %entry,
%for.body
//   %i.05 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
//   %sum.04 = phi i32 [ 0, %entry ], [ %add, %for.body ]
//   %idxprom = sext i32 %i.05 to i64
//   %arrayidx = getelementptr inbounds i32* %a, i64 %idxprom
//   %0 = load i32* %arrayidx, align 4, !tbaa !0
//   %add = add nsw i32 %0, %sum.04
//   %inc = add nsw i32 %i.05, 1
//   %cmp = icmp slt i32 %inc, 1000
//   br i1 %cmp, label %for.body, label %for.end
// *** IR Dump After Induction Variable Simplification ***
// for.body:                                         ; preds = %entry,
%for.body
//   %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
//   %sum.04 = phi i32 [ 0, %entry ], [ %add, %for.body ]
//   %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
//   %0 = load i32* %arrayidx, align 4, !tbaa !0
//   %add = add nsw i32 %0, %sum.04
//   %indvars.iv.next = add i64 %indvars.iv, 1
//   %lftr.wideiv = trunc i64 %indvars.iv.next to i32
//   %exitcond = icmp ne i32 %lftr.wideiv, 1000
//   br i1 %exitcond, label %for.body, label %for.end

You can see that %inc is transformed into %indvars.iv.next, and the nsw flag is
lost in the process.

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