[PATCH] D15827: [ValueTracking] fix bug computing isKnownToBeAPowerOfTwo() with arithmetic shift right (PR25900)

[Sanjay Patel via llvm-commits]

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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