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[ValueTracking] fix bug computing isKnownToBeAPowerOfTwo() with arithmetic shift right (PR25900) 

This is a fix for:
https://llvm.org/bugs/show_bug.cgi?id=25900

If we think that an arithmetic right shift of a power of two is always a power of two, 
an sdiv gets wrongly converted to udiv.

Differential Revision: http://reviews.llvm.org/D15827

llvm-svn: 256655
This commit is contained in:
Sanjay Patel 2015-12-30 22:40:52 +00:00
parent 934b092186
commit 41160c2094
2 changed files with 23 additions and 2 deletions
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5
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -1743,9 +1743,10 @@ bool isKnownToBeAPowerOfTwo(Value *V, bool OrZero, unsigned Depth,
return false;
Value *X = nullptr, *Y = nullptr;
// A shift of a power of two is a power of two or zero.
// A shift left or a logical shift right of a power of two is a power of two
// or zero.
if (OrZero && (match(V, m_Shl(m_Value(X), m_Value())) ||
match(V, m_Shr(m_Value(X), m_Value()))))
match(V, m_LShr(m_Value(X), m_Value()))))
return isKnownToBeAPowerOfTwo(X, /*OrZero*/ true, Depth, Q, DL);
if (ZExtInst *ZI = dyn_cast<ZExtInst>(V))

20
llvm/test/Analysis/ValueTracking/known-power-of-two.ll Normal file
View File

@ -0,0 +1,20 @@
; RUN: opt -S -instcombine < %s | FileCheck %s
; https://llvm.org/bugs/show_bug.cgi?id=25900
; An arithmetic shift right of a power of two is not a power
; of two if the original value is the sign bit. Therefore,
; we can't transform the sdiv into a udiv.
define i32 @pr25900(i32 %d) {
%and = and i32 %d, -2147483648
; The next 3 lines prevent another fold from masking the bug.
%ext = zext i32 %and to i64
%or = or i64 %ext, 4294967296
%trunc = trunc i64 %or to i32
%ashr = ashr exact i32 %trunc, 31
%div = sdiv i32 4, %ashr
ret i32 %div
; CHECK: sdiv
}