[PATCH] D107281: [SimpifyCFG] Speculate a store preceded by a local non-escaping load

[Momchil Velikov via Phabricator via llvm-commits]

chill updated this revision to Diff 363648.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107281/new/

https://reviews.llvm.org/D107281

Files:
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Transforms/SimplifyCFG/speculate-store.ll


Index: llvm/test/Transforms/SimplifyCFG/speculate-store.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/speculate-store.ll
+++ llvm/test/Transforms/SimplifyCFG/speculate-store.ll
@@ -175,6 +175,47 @@
   ret void
 }
 
+;; Speculate a store, preceded by a local, non-escaping load
+define i32 @f(i64 %i, i32 %b)  {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[A:%.*]] = alloca [2 x i32], align 8
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast [2 x i32]* [[A]] to i64*
+; CHECK-NEXT:    store i64 4294967296, i64* [[TMP0]], align 8
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [2 x i32], [2 x i32]* [[A]], i64 0, i64 [[I:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[TMP1]], [[B:%.*]]
+; CHECK-NEXT:    [[SPEC_STORE_SELECT:%.*]] = select i1 [[CMP]], i32 [[B]], i32 [[TMP1]]
+; CHECK-NEXT:    store i32 [[SPEC_STORE_SELECT]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[ARRAYIDX1:%.*]] = getelementptr inbounds [2 x i32], [2 x i32]* [[A]], i64 0, i64 0
+; CHECK-NEXT:    [[TMP2:%.*]] = load i32, i32* [[ARRAYIDX1]], align 4
+; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr inbounds [2 x i32], [2 x i32]* [[A]], i64 0, i64 1
+; CHECK-NEXT:    [[TMP3:%.*]] = load i32, i32* [[ARRAYIDX2]], align 4
+; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[TMP2]], [[TMP3]]
+; CHECK-NEXT:    ret i32 [[ADD]]
+;
+entry:
+  %a = alloca [2 x i32], align 8
+  %0 = bitcast [2 x i32]* %a to i64*
+  store i64 4294967296, i64* %0, align 8
+  %arrayidx = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 0, i64 %i
+  %1 = load i32, i32* %arrayidx, align 4
+  %cmp = icmp slt i32 %1, %b
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  store i32 %b, i32* %arrayidx, align 4
+  br label %if.end
+
+if.end:
+  %arrayidx1 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 0, i64 0
+  %2 = load i32, i32* %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 0, i64 1
+  %3 = load i32, i32* %arrayidx2, align 4
+  %add = add nsw i32 %2, %3
+  ret i32 %add
+}
+
 ; CHECK: !0 = !{!"branch_weights", i32 3, i32 5}
 !0 = !{!"branch_weights", i32 3, i32 5}
 
Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -25,6 +25,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/EHPersonalities.h"
 #include "llvm/Analysis/GuardUtils.h"
@@ -2250,6 +2251,19 @@
         return SI->getValueOperand();
       return nullptr; // Unknown store.
     }
+
+    if (auto *LI = dyn_cast<LoadInst>(&CurI)) {
+      if (LI->getPointerOperand() == StorePtr && LI->getType() == StoreTy &&
+          LI->isSimple()) {
+        // "Stack" memory is always writable, so we cannot trap. If the address
+        // does not escape, we cannot introduce a data race either.
+        auto *AI = dyn_cast<AllocaInst>(getUnderlyingObject(StorePtr));
+        if (AI && !PointerMayBeCaptured(AI, false, true))
+          // Found a previous load, return it.
+          return LI;
+      }
+      // The load didn't work out, but we may still find a store.
+    }
   }
 
   return nullptr;


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