GlobalsAA: Functions with the argmemonly attribute won't read arbitrary globals

[• tstellarAMD]

Summary

In preparation for changing GlobalsAA to stop assuming that intrinsics
can't read arbitrary globals, we need to make sure GlobalsAA is querying
function attributes rather than relying on this assumption.

This patch was inspired by: http://reviews.llvm.org/D20206

[• tstellarAMD]

Simplify the tests cases.

[eli.friedman]

For the testcase, maybe try passing "-disable-basicaa"? Or maybe come up with something BasicAA can't analyze, like a call to a function which calls an argmemonly function.

[• tstellarAMD]

Pass -disable-basicaa to opt. The testcase now fails with ToT, but passes with this patch.

[• tstellarAMD]

In D21318#458374, @eli.friedman wrote:

For the testcase, maybe try passing "-disable-basicaa"? Or maybe come up with something BasicAA can't analyze, like a call to a function which calls an argmemonly function.

Thanks for the tip. -disable-basicaa makes the testcase depend on this change now.

[• tstellarAMD]

Ping.

[hfinkel]

LGTM

llvm/trunk/lib/Analysis/GlobalsModRef.cpp

for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
       }
 
       if (F->isDeclaration()) {
         // Try to get mod/ref behaviour from function attributes.
         if (F->doesNotAccessMemory()) {
           // Can't do better than that!
         } else if (F->onlyReadsMemory()) {
           FI.addModRefInfo(MRI_Ref);
-          if (!F->isIntrinsic())
+          if (!F->isIntrinsic() && !F->onlyAccessesArgMemory())
             // This function might call back into the module and read a global -
             // consider every global as possibly being read by this function.
             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();

llvm/trunk/test/Analysis/GlobalsModRef/func-memattributes.ll

+; RUN: opt < %s -disable-basicaa -globals-aa -dse -S | FileCheck %s
+
+@X = internal global i32 4
+
+define void @test0() {
+; CHECK-LABEL: @test0
+; CHECK: store i32 0, i32* @X
+; CHECK-NEXT: call void @func_readonly() #0
+; CHECK-NEXT: store i32 1, i32* @X
+  store i32 0, i32* @X
+  call void @func_readonly() #0
+  store i32 1, i32* @X
+  ret void
+}
+
+define void @test1() {
+; CHECK-LABEL: @test1
+; CHECK-NOT: store
+; CHECK: call void @func_read_argmem_only() #1
+; CHECK-NEXT: store i32 3, i32* @X
+  store i32 2, i32* @X
+  call void @func_read_argmem_only() #1
+  store i32 3, i32* @X
+  ret void
+}
+
+declare void @func_readonly() #0
+declare void @func_read_argmem_only() #1
+
+attributes #0 = { readonly }
+attributes #1 = { readonly argmemonly }