[PATCH] llvm.noalias - don't block EarlyCSE

[hfinkel at anl.gov]

Hi chandlerc, reames,

This part of the series started by D9375, and as with llvm.assume, prevents llvm.noalias from forming a generation barrier in EarlyCSE.

http://reviews.llvm.org/D9377

Files:
  lib/Transforms/Scalar/EarlyCSE.cpp
  test/Transforms/EarlyCSE/basic.ll

Index: lib/Transforms/Scalar/EarlyCSE.cpp
===================================================================
--- lib/Transforms/Scalar/EarlyCSE.cpp
+++ lib/Transforms/Scalar/EarlyCSE.cpp
@@ -487,9 +487,10 @@
     // Skip assume intrinsics, they don't really have side effects (although
     // they're marked as such to ensure preservation of control dependencies),
     // and this pass will not disturb any of the assumption's control
-    // dependencies.
-    if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
-      DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
+    // dependencies. Likewise, noalias intrinsics don't actually write.
+    if (match(Inst, m_CombineOr(m_Intrinsic<Intrinsic::noalias>(),
+                                m_Intrinsic<Intrinsic::assume>()))) {
+      DEBUG(dbgs() << "EarlyCSE skipping intrinsic: " << *Inst << '\n');
       continue;
     }
 
Index: test/Transforms/EarlyCSE/basic.ll
===================================================================
--- test/Transforms/EarlyCSE/basic.ll
+++ test/Transforms/EarlyCSE/basic.ll
@@ -54,6 +54,16 @@
   ; CHECK: ret i32 0
 }
 
+; CHECK-LABEL: @test2b(
+define i32 @test2b(i32 *%P, i1 %b) {
+  %V1 = load i32, i32* %P
+  call i8* @llvm.noalias.p0i8(i8* undef, metadata !1)
+  %V2 = load i32, i32* %P
+  %Diff = sub i32 %V1, %V2
+  ret i32 %Diff
+  ; CHECK: ret i32 0
+}
+
 ;; Cross block load value numbering.
 ; CHECK-LABEL: @test3(
 define i32 @test3(i32 *%P, i1 %Cond) {
@@ -134,6 +144,15 @@
   ; CHECK: ret i32 42
 }
 
+; CHECK-LABEL: @test6b(
+define i32 @test6b(i32 *%P, i1 %b) {
+  store i32 42, i32* %P
+  call i8* @llvm.noalias.p0i8(i8* undef, metadata !1)
+  %V1 = load i32, i32* %P
+  ret i32 %V1
+  ; CHECK: ret i32 42
+}
+
 ;; Trivial dead store elimination.
 ; CHECK-LABEL: @test7(
 define void @test7(i32 *%P) {
@@ -203,3 +222,9 @@
   ; CHECK: load i32, i32* %P1
   ; CHECK: load i32, i32* %P1
 }
+
+declare i8* @llvm.noalias.p0i8(i8*, metadata) nounwind
+
+!0 = !{!0, !"some domain"}
+!1 = !{!1, !0, !"some scope"}
+

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