[llvm] 4de3184 - [LAA] Use cross-iteration alias analysis

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2022-12-05T09:27:13+01:00
New Revision: 4de3184f07fd8c548125d315dd306d4afa7c9698

URL: https://github.com/llvm/llvm-project/commit/4de3184f07fd8c548125d315dd306d4afa7c9698
DIFF: https://github.com/llvm/llvm-project/commit/4de3184f07fd8c548125d315dd306d4afa7c9698.diff

LOG: [LAA] Use cross-iteration alias analysis

LAA analyzes cross-iteration memory dependencies, as such AA should
not make assumptions about equality of values inside the loop, as
they may come from different iterations.

Fix this by exposing the MayBeCrossIteration AA flag and enabling
it for LAA.

Differential Revision: https://reviews.llvm.org/D137958

Added: 
    llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll

Modified: 
    llvm/include/llvm/Analysis/AliasAnalysis.h
    llvm/lib/Analysis/LoopAccessAnalysis.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Analysis/AliasAnalysis.h b/llvm/include/llvm/Analysis/AliasAnalysis.h
index c50cb2d202118..99d10cba6b5bb 100644
--- a/llvm/include/llvm/Analysis/AliasAnalysis.h
+++ b/llvm/include/llvm/Analysis/AliasAnalysis.h
@@ -651,6 +651,11 @@ class BatchAAResults {
                                 DominatorTree *DT) {
     return AA.callCapturesBefore(I, MemLoc, DT, AAQI);
   }
+
+  /// Assume that values may come from 
diff erent cycle iterations.
+  void enableCrossIterationMode() {
+    AAQI.MayBeCrossIteration = true;
+  }
 };
 
 /// Temporary typedef for legacy code that uses a generic \c AliasAnalysis

diff  --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 1bc2f6b5f940c..4e688563e6ded 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -622,7 +622,10 @@ class AccessAnalysis {
   AccessAnalysis(Loop *TheLoop, AAResults *AA, LoopInfo *LI,
                  MemoryDepChecker::DepCandidates &DA,
                  PredicatedScalarEvolution &PSE)
-      : TheLoop(TheLoop), BAA(*AA), AST(BAA), LI(LI), DepCands(DA), PSE(PSE) {}
+      : TheLoop(TheLoop), BAA(*AA), AST(BAA), LI(LI), DepCands(DA), PSE(PSE) {
+    // We're analyzing dependences across loop iterations.
+    BAA.enableCrossIterationMode();
+  }
 
   /// Register a load  and whether it is only read from.
   void addLoad(MemoryLocation &Loc, Type *AccessTy, bool IsReadOnly) {

diff  --git a/llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll b/llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll
new file mode 100644
index 0000000000000..07e32f443554b
--- /dev/null
+++ b/llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll
@@ -0,0 +1,30 @@
+; RUN: opt -passes='print<access-info>' -disable-output 2>&1 < %s | FileCheck %s
+
+; CHECK: Dependences:
+; CHECK-NEXT: Unknown:
+; CHECK-NEXT: %t63 = load double, ptr %t62, align 8 ->
+; CHECK-NEXT: store double %t63, ptr %t64, align 8
+
+define i32 @test() {
+   %a1 = alloca [128 x double], align 8
+   %a2 = alloca [128 x double], align 8
+   %a3 = alloca [128 x double], align 8
+   %t30 = getelementptr double, ptr %a2, i64 -32
+   br label %loop
+
+loop:
+   %t58 = phi i64 [ %t65, %loop ], [ 0, %0 ]
+   %t59 = icmp ule i64 %t58, 32
+   %t60 = select i1 %t59, ptr %a1, ptr %t30
+   %t62 = getelementptr inbounds double, ptr %t60, i64 %t58
+   %t63 = load double, ptr %t62, align 8
+   %t61 = select i1 %t59, ptr %a2, ptr %a3
+   %t64 = getelementptr inbounds double, ptr %t61, i64 %t58
+   store double %t63, ptr %t64, align 8
+   %t65 = add nuw nsw i64 %t58, 1
+   %t66 = icmp eq i64 %t65, 94
+   br i1 %t66, label %exit, label %loop
+
+exit:
+   ret i32 0
+}