[PATCH] D15827: [ValueTracking] fix bug computing isKnownToBeAPowerOfTwo() with arithmetic shift right (PR25900)
Sanjay Patel via llvm-commits
llvm-commits at lists.llvm.org
Wed Dec 30 11:50:25 PST 2015
spatel created this revision.
spatel added reviewers: sanjoy, hfinkel, majnemer.
spatel added a subscriber: llvm-commits.
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.
I didn't see a way to simplify the test case further.
http://reviews.llvm.org/D15827
Files:
lib/Analysis/ValueTracking.cpp
test/Analysis/ValueTracking/known-power-of-two.ll
Index: test/Analysis/ValueTracking/known-power-of-two.ll
===================================================================
--- test/Analysis/ValueTracking/known-power-of-two.ll
+++ test/Analysis/ValueTracking/known-power-of-two.ll
@@ -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
+}
+
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -1743,9 +1743,12 @@
return false;
Value *X = nullptr, *Y = nullptr;
- // A shift of a power of two is a power of two or zero.
+ // A left shift or a logical shift right of a power of two is a power of two
+ // or zero.
+ // TODO: An arithmetic shift right of a power of two is a power of two or zero
+ // if the value is not the sign bit (minimum signed value).
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))
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