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InstCombine: Don't miscompile (x lshr C1) udiv C2

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We have a transform that changes:
  (x lshr C1) udiv C2
into:
  x udiv (C2 << C1)

However, it is unsafe to do so if C2 << C1 discards any of C2's bits.

This fixes PR21255.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219634 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-10-13 21:48:30 +00:00
parent 2061a56b8a
commit dfe81adbce
4 changed files with 40 additions and 13 deletions
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3
include/llvm/ADT/APInt.h
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@@ -945,7 +945,8 @@ public:
APInt sdiv_ov(const APInt &RHS, bool &Overflow) const;
APInt smul_ov(const APInt &RHS, bool &Overflow) const;
APInt umul_ov(const APInt &RHS, bool &Overflow) const;
APInt sshl_ov(unsigned Amt, bool &Overflow) const;
APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
/// \brief Array-indexing support.
///