[llvm] 9bc7e54 - [BasicAA] Make range check more precise

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2021-10-27T12:40:58+02:00
New Revision: 9bc7e543b4ef5eef2be2325204db20073fc18e36

URL: https://github.com/llvm/llvm-project/commit/9bc7e543b4ef5eef2be2325204db20073fc18e36
DIFF: https://github.com/llvm/llvm-project/commit/9bc7e543b4ef5eef2be2325204db20073fc18e36.diff

LOG: [BasicAA] Make range check more precise

Make the range check more precise by calculating the range of
potentially accessed bytes for both accesses and checking whether
their intersection is empty. In that case there can be no overlap
between the accesses and the result is NoAlias.

This is more powerful than the previous approach, because it can
deal with sign-wrapped ranges. In the test case the original range
is [-1, INT_MAX] but becomes [0, INT_MIN] after applying the offset.
This is a wrapping range, so getSignedMin/getSignedMax will treat
it as a full range. However, the range excludes the elements
[INT_MIN+1, -1], which is enough to prove NoAlias with an access
at offset -1.

Differential Revision: https://reviews.llvm.org/D112486

Added: 
    

Modified: 
    llvm/lib/Analysis/BasicAliasAnalysis.cpp
    llvm/test/Analysis/BasicAA/range.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index f0e5d65f961d1..0305732ca5d55 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1252,7 +1252,7 @@ AliasResult BasicAAResult::aliasGEP(
     APInt GCD;
     bool AllNonNegative = DecompGEP1.Offset.isNonNegative();
     bool AllNonPositive = DecompGEP1.Offset.isNonPositive();
-    ConstantRange Range = ConstantRange(DecompGEP1.Offset);
+    ConstantRange OffsetRange = ConstantRange(DecompGEP1.Offset);
     for (unsigned i = 0, e = DecompGEP1.VarIndices.size(); i != e; ++i) {
       const VariableGEPIndex &Index = DecompGEP1.VarIndices[i];
       const APInt &Scale = Index.Scale;
@@ -1277,12 +1277,12 @@ AliasResult BasicAAResult::aliasGEP(
                           (SignKnownOne && Scale.isNonNegative());
       }
 
-      assert(Range.getBitWidth() == Scale.getBitWidth() &&
+      assert(OffsetRange.getBitWidth() == Scale.getBitWidth() &&
              "Bit widths are normalized to MaxPointerSize");
-      Range = Range.add(Index.Val
+      OffsetRange = OffsetRange.add(Index.Val
                             .evaluateWith(computeConstantRange(
                                 Index.Val.V, true, &AC, Index.CxtI))
-                            .sextOrTrunc(Range.getBitWidth())
+                            .sextOrTrunc(OffsetRange.getBitWidth())
                             .smul_fast(ConstantRange(Scale)));
     }
 
@@ -1315,15 +1315,15 @@ AliasResult BasicAAResult::aliasGEP(
         (-DecompGEP1.Offset).uge(V1Size.getValue()))
       return AliasResult::NoAlias;
 
-    if (!Range.isEmptySet()) {
-      // We know that Offset >= MinOffset.
-      // (MinOffset >= V2Size) => (Offset >= V2Size) => NoAlias.
-      if (V2Size.hasValue() && Range.getSignedMin().sge(V2Size.getValue()))
-        return AliasResult::NoAlias;
-
-      // We know that Offset <= MaxOffset.
-      // (MaxOffset <= -V1Size) => (Offset <= -V1Size) => NoAlias.
-      if (V1Size.hasValue() && Range.getSignedMax().sle(-V1Size.getValue()))
+    if (V1Size.hasValue() && V2Size.hasValue()) {
+      // Compute ranges of potentially accessed bytes for both accesses. If the
+      // interseciton is empty, there can be no overlap.
+      unsigned BW = OffsetRange.getBitWidth();
+      ConstantRange Range1 = OffsetRange.add(
+          ConstantRange(APInt(BW, 0), APInt(BW, V1Size.getValue())));
+      ConstantRange Range2 =
+          ConstantRange(APInt(BW, 0), APInt(BW, V2Size.getValue()));
+      if (Range1.intersectWith(Range2).isEmptySet())
         return AliasResult::NoAlias;
     }
 

diff  --git a/llvm/test/Analysis/BasicAA/range.ll b/llvm/test/Analysis/BasicAA/range.ll
index 6bdb7ccb16e2a..895332086f78d 100644
--- a/llvm/test/Analysis/BasicAA/range.ll
+++ b/llvm/test/Analysis/BasicAA/range.ll
@@ -196,13 +196,13 @@ define void @multiple(i32* %p, i32* %o1_ptr, i32* %o2_ptr) {
   ret void
 }
 
-; TODO: p.neg1 and p.o.1 don't alias, even though the addition o+1 may overflow.
+; p.neg1 and p.o.1 don't alias, even though the addition o+1 may overflow.
 ; While it makes INT_MIN a possible offset, offset -1 is not possible.
 ; CHECK-LABEL: Function: benign_overflow
 ; CHECK: MayAlias: i8* %p, i8* %p.o
 ; CHECK: MayAlias: i8* %p.neg1, i8* %p.o
 ; CHECK: MayAlias: i8* %p, i8* %p.o.1
-; CHECK: MayAlias: i8* %p.neg1, i8* %p.o.1
+; CHECK: NoAlias: i8* %p.neg1, i8* %p.o.1
 ; CHECK: NoAlias:  i8* %p.o, i8* %p.o.1
 define void @benign_overflow(i8* %p, i64 %o) {
   %c = icmp sge i64 %o, -1