[llvm] efb8e15 - [DSE, MemorySSA] Add an early check for read clobbers to traversal.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2020-09-07T23:22:10+01:00
New Revision: efb8e156daa120a25f993b3142ef8d6ef766df5a

URL: https://github.com/llvm/llvm-project/commit/efb8e156daa120a25f993b3142ef8d6ef766df5a
DIFF: https://github.com/llvm/llvm-project/commit/efb8e156daa120a25f993b3142ef8d6ef766df5a.diff

LOG: [DSE,MemorySSA] Add an early check for read clobbers to traversal.

Depending on the benchmark, this early exit can save a substantial
amount of compile-time:

http://llvm-compile-time-tracker.com/compare.php?from=505f2d817aa8e07ba98e5fd4a8f6ff0666f89df1&to=eb4e441147f9b4b7a5fcbbc57428cadbe9e01f10&stat=instructions

Added: 
    llvm/test/Transforms/DeadStoreElimination/MSSA/read-clobber-after-overwrite.ll

Modified: 
    llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 109e15d6d7cf..49e811b298a6 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1901,6 +1901,18 @@ struct DSEState {
         return None;
       }
 
+      // Quick check if there are direct uses that are read-clobbers.
+      if (any_of(Current->uses(), [this, &DefLoc, StartAccess](Use &U) {
+            if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(U.getUser()))
+              return !MSSA.dominates(StartAccess, UseOrDef) &&
+                     isReadClobber(DefLoc, UseOrDef->getMemoryInst());
+            return false;
+          })) {
+        Cache.KnownReads.insert(Current);
+        LLVM_DEBUG(dbgs() << "   ...  found a read clobber\n");
+        return None;
+      }
+
       // If Current cannot be analyzed or is not removable, check the next
       // candidate.
       if (!hasAnalyzableMemoryWrite(CurrentI, TLI) || !isRemovable(CurrentI)) {

diff  --git a/llvm/test/Transforms/DeadStoreElimination/MSSA/read-clobber-after-overwrite.ll b/llvm/test/Transforms/DeadStoreElimination/MSSA/read-clobber-after-overwrite.ll
new file mode 100644
index 000000000000..4f704c35a90b
--- /dev/null
+++ b/llvm/test/Transforms/DeadStoreElimination/MSSA/read-clobber-after-overwrite.ll
@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -dse -enable-dse-memoryssa -S %s | FileCheck %s
+
+declare i1 @cond() readnone
+
+define i32 @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[M0:%.*]] = alloca [4 x i32], align 16
+; CHECK-NEXT:    br label [[LOOP_1:%.*]]
+; CHECK:       loop.1:
+; CHECK-NEXT:    br label [[LOOP_2:%.*]]
+; CHECK:       loop.2:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[LOOP_1]] ], [ [[IV_NEXT:%.*]], [[LOOP_2]] ]
+; CHECK-NEXT:    [[PTR_1:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[M0]], i64 3, i64 [[IV]]
+; CHECK-NEXT:    [[PTR_2:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[M0]], i64 0, i64 [[IV]]
+; CHECK-NEXT:    store i32 20, i32* [[PTR_2]], align 4
+; CHECK-NEXT:    store i32 30, i32* [[PTR_1]], align 4
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[C_3:%.*]] = call i1 @cond()
+; CHECK-NEXT:    br i1 [[C_3]], label [[LOOP_1_LATCH:%.*]], label [[LOOP_2]]
+; CHECK:       loop.1.latch:
+; CHECK-NEXT:    [[C_2:%.*]] = call i1 @cond()
+; CHECK-NEXT:    br i1 [[C_2]], label [[EXIT:%.*]], label [[LOOP_1]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[PTR_3:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[M0]], i64 0, i64 1
+; CHECK-NEXT:    [[LV:%.*]] = load i32, i32* [[PTR_3]], align 16
+; CHECK-NEXT:    ret i32 [[LV]]
+;
+entry:
+  %M0 = alloca [4 x i32], align 16
+  br label %loop.1
+
+loop.1:
+  br label %loop.2
+
+loop.2:
+  %iv = phi i64 [ 0, %loop.1 ], [ %iv.next, %loop.2 ]
+  %ptr.1 = getelementptr inbounds [4 x i32], [4 x i32]* %M0, i64 3, i64 %iv
+  store i32 10, i32* %ptr.1, align 4
+  %ptr.2 = getelementptr inbounds [4 x i32], [4 x i32]* %M0, i64 0, i64 %iv
+  store i32 20, i32* %ptr.2, align 4
+  store i32 30, i32* %ptr.1, align 4
+  %iv.next = add nuw nsw i64 %iv, 1
+  %c.3 = call i1 @cond()
+  br i1 %c.3, label %loop.1.latch, label %loop.2
+
+loop.1.latch:
+  %c.2 = call i1 @cond()
+  br i1 %c.2, label %exit, label %loop.1
+
+exit:
+  %ptr.3 = getelementptr inbounds [4 x i32], [4 x i32]* %M0, i64 0, i64 1
+  %lv = load i32, i32* %ptr.3, align 16
+  ret i32 %lv
+
+
+}