[PATCH] D20206: GlobalsAA: Don't assume that intrinsics can't access global variables

[Tom Stellard via llvm-commits]

tstellarAMD created this revision.
tstellarAMD added a reviewer: hfinkel.
tstellarAMD added a subscriber: llvm-commits.

The Gobals Alias Analysis was assuming that intrinsics could never read
or write global variables unless the variable was passed as one of the
arguments to the intrinsic.

I can't find this property of intrinsics documented anywhere, so the
code has been updated to treat intrinsics the same as normal function
calls when analyzing usage of global variables.

This fixes a bug in the AMDGPU backend where this incorrect analysis
was causing to the GVN pass to hoist a load across a barrier intrinsic.

http://reviews.llvm.org/D20206

Files:
  lib/Analysis/GlobalsModRef.cpp
  test/Analysis/GlobalsModRef/intrinsic-global.ll

Index: test/Analysis/GlobalsModRef/intrinsic-global.ll
===================================================================
--- /dev/null
+++ test/Analysis/GlobalsModRef/intrinsic-global.ll
@@ -0,0 +1,34 @@
+; RUN: opt < %s -globals-aa -gvn -S | FileCheck %s
+
+; Check that gvn will not hoist the load past the @llvm.amdgcn.s.barrier
+; intrinsic, which should be assumed to read/write all global variables
+; since it does not have any attributes that describe its interaction with
+; memory.
+
+ at global = internal  addrspace(3) global i32 undef, align 4
+ at global.1 = internal  addrspace(3) global i32 undef, align 4
+
+; CHECK-LABEL: @widget
+; CHECK: bb5:
+; CHECK-NEXT: call void @llvm.amdgcn.s.barrier()
+; CHECK-NEXT: %tmp6 = load i32, i32 addrspace(3)* @global.1
+define void @widget(i32 %arg, i32 %arg1, i32 %arg2) #0 {
+bb:
+  %tmp3 = icmp eq i32 %arg2, 0
+  br i1 %tmp3, label %bb4, label %bb5
+
+bb4:                                              ; preds = %bb
+  store i32 0, i32 addrspace(3)* @global.1, align 4
+  br label %bb5
+
+bb5:                                              ; preds = %bb4, %bb
+  call void @llvm.amdgcn.s.barrier() #0
+  %tmp6 = load i32, i32 addrspace(3)* @global.1, align 4
+  store i32 %tmp6, i32 addrspace(3)* @global, align 4
+  ret void
+}
+
+; Function Attrs: convergent nounwind
+declare void @llvm.amdgcn.s.barrier() #0
+
+attributes #0 = { convergent nounwind }
Index: lib/Analysis/GlobalsModRef.cpp
===================================================================
--- lib/Analysis/GlobalsModRef.cpp
+++ lib/Analysis/GlobalsModRef.cpp
@@ -504,9 +504,7 @@
             FI.setMayReadAnyGlobal();
         } else {
           FI.addModRefInfo(MRI_ModRef);
-          // Can't say anything useful unless it's an intrinsic - they don't
-          // read or write global variables of the kind considered here.
-          KnowNothing = !F->isIntrinsic();
+          KnowNothing = true;
         }
         continue;
       }


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