[BasicAA] Revert "Revert r218714 - Make better use of zext and sign information."

This reverts r218944, which reverted r218714, plus a bug fix.

Description of the bug in r218714 (by Nick)

The original patch forgot to check if the Scale in VariableGEPIndex flipped the
sign of the variable. The BasicAA pass iterates over the instructions in the
order they appear in the function, and so BasicAliasAnalysis::aliasGEP is
called with the variable it first comes across as parameter GEP1. Adding a
%reorder label puts the definition of %a after %b so aliasGEP is called with %b
as the first parameter and %a as the second. aliasGEP later calculates that %a
== %b + 1 - %idxprom where %idxprom >= 0 (if %a was passed as the first
parameter it would calculate %b == %a - 1 + %idxprom where %idxprom >= 0) -
ignoring that %idxprom is scaled by -1 here lead the patch to incorrectly
conclude that %a > %b.

Revised patch by Nick White, thanks! Thanks to Lang to isolating the bug.
Slightly modified by me to add an early exit from the loop and avoid
unnecessary, but expensive, function calls.

Original commit message:

Two related things:

 1. Fixes a bug when calculating the offset in GetLinearExpression. The code
    previously used zext to extend the offset, so negative offsets were converted
    to large positive ones.

 2. Enhance aliasGEP to deduce that, if the difference between two GEP
    allocations is positive and all the variables that govern the offset are also
    positive (i.e. the offset is strictly after the higher base pointer), then
    locations that fit in the gap between the two base pointers are NoAlias.

Patch by Nick White!

llvm-svn: 219135

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -254,7 +254,10 @@ static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
     Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
                                         DL, Depth+1, AT, DT);
     Scale = Scale.zext(OldWidth);
-    Offset = Offset.zext(OldWidth);
+
+    // We have to sign-extend even if Extension == EK_ZeroExt as we can't
+    // decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
+    Offset = Offset.sext(OldWidth);
 
     return Result;
   }
@@ -1055,8 +1058,38 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
   // Grab the least significant bit set in any of the scales.
   if (!GEP1VariableIndices.empty()) {
     uint64_t Modulo = 0;
-    for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
+    bool AllPositive = true;
+    for (unsigned i = 0, e = GEP1VariableIndices.size();
+         i != e && AllPositive; ++i) {
+      const Value *V = GEP1VariableIndices[i].V;
       Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
+
+      bool SignKnownZero, SignKnownOne;
+      ComputeSignBit(
+        const_cast<Value *>(V),
+        SignKnownZero, SignKnownOne,
+        DL, 0, AT, nullptr, DT);
+
+      // Zero-extension widens the variable, and so forces the sign
+      // bit to zero.
+      bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
+      SignKnownZero |= IsZExt;
+      SignKnownOne &= !IsZExt;
+
+      // If the variable begins with a zero then we know it's
+      // positive, regardless of whether the value is signed or
+      // unsigned.
+      int64_t Scale = GEP1VariableIndices[i].Scale;
+      AllPositive &= 
+        (SignKnownZero && Scale >= 0) ||
+        (SignKnownOne && Scale < 0);
+
+      // If the Value is currently positive but could change in a cycle,
+      // then we can't guarantee it'll always br positive.
+      if (AllPositive && !isValueEqualInPotentialCycles(V, V))
+        AllPositive = false;
+    }
+
     Modulo = Modulo ^ (Modulo & (Modulo - 1));
 
     // We can compute the difference between the two addresses
@@ -1066,6 +1099,12 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
     if (V1Size != UnknownSize && V2Size != UnknownSize &&
         ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
       return NoAlias;
+
+    // If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
+    // If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
+    // don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
+    if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
+      return NoAlias;
   }
 
   // Statically, we can see that the base objects are the same, but the

llvm/test/Analysis/BasicAA/phi-aa.ll

@@ -39,6 +39,7 @@ return:
 
 ; CHECK-LABEL: pr18068
 ; CHECK: MayAlias: i32* %0, i32* %arrayidx5
+; CHECK: NoAlias: i32* %arrayidx13, i32* %arrayidx5
 
 define i32 @pr18068(i32* %jj7, i32* %j) {
 entry:

llvm/test/Analysis/BasicAA/zext.ll

@@ -0,0 +1,87 @@
+; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: test_with_zext
+; CHECK:  NoAlias: i8* %a, i8* %b
+
+define void @test_with_zext() {
+  %1 = tail call i8* @malloc(i64 120)
+  %a = getelementptr inbounds i8* %1, i64 8
+  %2 = getelementptr inbounds i8* %1, i64 16
+  %3 = zext i32 3 to i64
+  %b = getelementptr inbounds i8* %2, i64 %3
+  ret void
+}
+
+; CHECK-LABEL: test_with_lshr
+; CHECK:  NoAlias: i8* %a, i8* %b
+
+define void @test_with_lshr(i64 %i) {
+  %1 = tail call i8* @malloc(i64 120)
+  %a = getelementptr inbounds i8* %1, i64 8
+  %2 = getelementptr inbounds i8* %1, i64 16
+  %3 = lshr i64 %i, 2
+  %b = getelementptr inbounds i8* %2, i64 %3
+  ret void
+}
+
+; CHECK-LABEL: test_with_a_loop
+; CHECK:  NoAlias: i8* %a, i8* %b
+
+define void @test_with_a_loop() {
+  %1 = tail call i8* @malloc(i64 120)
+  %a = getelementptr inbounds i8* %1, i64 8
+  %2 = getelementptr inbounds i8* %1, i64 16
+  br label %for.loop
+
+for.loop:
+  %i = phi i32 [ 0, %0 ], [ %i.next, %for.loop ]
+  %3 = zext i32 %i to i64
+  %b = getelementptr inbounds i8* %2, i64 %3
+  %i.next = add nuw nsw i32 %i, 1
+  %4 = icmp eq i32 %i.next, 10
+  br i1 %4, label %for.loop.exit, label %for.loop
+
+for.loop.exit:
+  ret void
+}
+
+; CHECK-LABEL: test_sign_extension
+; CHECK:  PartialAlias: i64* %b.i64, i8* %a
+
+define void @test_sign_extension(i32 %p) {
+  %1 = tail call i8* @malloc(i64 120)
+  %p.64 = zext i32 %p to i64
+  %a = getelementptr inbounds i8* %1, i64 %p.64
+  %p.minus1 = add i32 %p, -1
+  %p.minus1.64 = zext i32 %p.minus1 to i64
+  %b.i8 = getelementptr inbounds i8* %1, i64 %p.minus1.64
+  %b.i64 = bitcast i8* %b.i8 to i64*
+  ret void
+}
+
+; CHECK-LABEL: test_fe_tools
+; CHECK:  PartialAlias: i32* %a, i32* %b
+
+define void @test_fe_tools([8 x i32]* %values) {
+  br label %reorder
+
+for.loop:
+  %i = phi i32 [ 0, %reorder ], [ %i.next, %for.loop ]
+  %idxprom = zext i32 %i to i64
+  %b = getelementptr inbounds [8 x i32]* %values, i64 0, i64 %idxprom
+  %i.next = add nuw nsw i32 %i, 1
+  %1 = icmp eq i32 %i.next, 10
+  br i1 %1, label %for.loop.exit, label %for.loop
+
+reorder:
+  %a = getelementptr inbounds [8 x i32]* %values, i64 0, i64 1
+  br label %for.loop
+
+for.loop.exit:
+  ret void
+}
+
+; Function Attrs: nounwind
+declare noalias i8* @malloc(i64)