From: Philip Reames <listmail@philipreames.com>
Date: Tue, 18 Nov 2014 17:46:32 +0000 (+0000)
Subject: Tweak EarlyCSE to recognize series of dead stores
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=0814bd85dddc576621a903b78301463fb22c8c0b;p=llvm

Tweak EarlyCSE to recognize series of dead stores

EarlyCSE is giving up on the current instruction immediately when it recognizes that the current instruction makes a previous store trivially dead. There's no reason to do this. Once the previous store has been deleted, it's perfectly legal to remember the value of the current store (for value forwarding) and the fact the store occurred (it could be dead too!).

Reviewed by: Hal
Differential Revision: http://reviews.llvm.org/D6301



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222241 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/EarlyCSE.cpp b/lib/Transforms/Scalar/EarlyCSE.cpp
index 6f166b8ee7f..cd2ecad4a18 100644
--- a/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -546,7 +546,7 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
           Changed = true;
           ++NumDSE;
           LastStore = nullptr;
-          continue;
+          // fallthrough - we can exploit information about this store
         }
 
         // Okay, we just invalidated anything we knew about loaded values.  Try
diff --git a/test/Transforms/EarlyCSE/basic.ll b/test/Transforms/EarlyCSE/basic.ll
index 9f3e474aff9..155d36f60e2 100644
--- a/test/Transforms/EarlyCSE/basic.ll
+++ b/test/Transforms/EarlyCSE/basic.ll
@@ -154,4 +154,42 @@ define i32 @test8(i32 *%P) {
   ; CHECK: ret i32 0
 }
 
+;; Trivial DSE can't be performed across a readonly call.  The call
+;; can observe the earlier write.
+; CHECK-LABEL: @test9(
+define i32 @test9(i32 *%P) {
+  store i32 4, i32* %P
+  %V1 = call i32 @func(i32* %P) readonly
+  store i32 5, i32* %P        
+  ret i32 %V1
+  ; CHECK: store i32 4, i32* %P        
+  ; CHECK-NEXT: %V1 = call i32 @func(i32* %P)
+  ; CHECK-NEXT: store i32 5, i32* %P        
+  ; CHECK-NEXT: ret i32 %V1
+}
+
+;; Trivial DSE can be performed across a readnone call.
+; CHECK-LABEL: @test10
+define i32 @test10(i32 *%P) {
+  store i32 4, i32* %P
+  %V1 = call i32 @func(i32* %P) readnone
+  store i32 5, i32* %P        
+  ret i32 %V1
+  ; CHECK-NEXT: %V1 = call i32 @func(i32* %P)
+  ; CHECK-NEXT: store i32 5, i32* %P        
+  ; CHECK-NEXT: ret i32 %V1
+}
+
+;; Trivial dead store elimination - should work for an entire series of dead stores too.
+; CHECK-LABEL: @test11(
+define void @test11(i32 *%P) {
+  store i32 42, i32* %P
+  store i32 43, i32* %P
+  store i32 44, i32* %P
+  store i32 45, i32* %P
+  ret void
+  ; CHECK-NEXT: store i32 45
+  ; CHECK-NEXT: ret void
+}
+