[llvm-commits] PATCH: Preserving the 'nsw' flag in the instruction combiner.

[Pranav Bhandarkar]

Hi,

Subsequent to a discussion on the 'nsw' flag on llvm dev
(http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-June/040856.html), I have
written a small function that allows the instruction combiner to preserve
the 'nsw' flag under certain conditions. Preserving this flag allows
optimization passes down the pipeline to be more aggressive.

For instance, consider this synthetic test case.
int foo (int x)
{     int j = x + 2;
      int y = j + 3;

      if (j < y) return 1;
      else return 0;
}
This can be reduced to always return 1. However LLVM does not do this
transformation. Consider the following piece of code

1 %add = add nsw i32 %tmp, 2
2 %add3 = add nsw i32 %add, 3
3 %cmp = icmp slt i32 %add, %add3
4 br i1 %cmp, label %if.then, label %if.else

Here add = tmp + 2 and add3 is effectively tmp + 5 and the comparison "slt
%add, %add3" is always true. However, the instruction combiner combines
instructions 1 and 2 to produce

2 %add3 = add i32 %tmp, 5

Note the 'nsw' flag is dropped because the combiner does not reason about
the applicability of the 'nsw' flag in the new combined instruction. If now
the simplifycfg pass is run it does not simplify the comparison because one
of its operands has the 'nsw' flag while the other does not. The attached
patch checks the resulting constant in the new instruction for overflow. If
not, it keeps the flag. This allows the "simplifycfg" pass to simplify the
comparison and reduces the above function to return 1.

I have three attachments - The patch, a testcase that should be added to
test/Transforms/InstCombine and my regression report. This patch was
successfully regression tested on x86_64.

Pranav
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