[AST] Mark invariant.starts as being readonly

[reames]

Summary

These intrinsics are modelled as writing for control flow purposes, but they don't actually write to any location. Marking these - as we did for guards - allows LICM to hoist loads out of loops containing invariant.starts.

[reames]

test/Transforms/LICM/invariant.start.ll

89

This has been addressed in a separate change and will disappear when rebased.

[anna]

LGTM.

lib/Analysis/AliasSetTracker.cpp

void AliasSet::addUnknownInst(Instruction *I, AliasAnalysis &AA) {
   if (UnknownInsts.empty())
     addRef();
   UnknownInsts.emplace_back(I);
 
   // Guards are marked as modifying memory for control flow modelling purposes,
   // but don't actually modify any specific memory location.
   using namespace PatternMatch;
   bool MayWriteMemory = I->mayWriteToMemory() &&
-                        !match(I, m_Intrinsic<Intrinsic::experimental_guard>());
+    !match(I, m_Intrinsic<Intrinsic::experimental_guard>()) &&
+    !(I->use_empty() && match(I, m_Intrinsic<Intrinsic::invariant_start>()));
   if (!MayWriteMemory) {
     Alias = SetMayAlias;
     Access |= RefAccess;
     return;
   }
 
   // FIXME: This should use mod/ref information to make this not suck so bad
   Alias = SetMayAlias;

test/Transforms/LICM/invariant.start.ll

 ; RUN: opt -licm -basicaa < %s -S | FileCheck %s
 ; RUN: opt -aa-pipeline=basic-aa -passes='require<aa>,require<targetir>,require<scalar-evolution>,require<opt-remark-emit>,loop(licm)' < %s -S | FileCheck %s
 
-; TODO: should be able to hoist both load and invariant.start
+; TODO: should be able to hoist the invariant.start
 define void @test1(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @test1(
 ; CHECK-LABEL: entry:
+; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
 ; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
-; CHECK: %val = load i32, i32* %ptr
 
 entry:
   br label %loop
 
 loop:
   %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
   call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
   %val = load i32, i32* %ptr
   %x.inc = add i32 %x, %val
   br label %loop
 }
 
-;; TODO: despite the loop varying invariant.start, we should be
-;; able to hoist the load
+;; despite the loop varying invariant.start, we can hoist the load
 define void @test2(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @test2(
 ; CHECK-LABEL: entry:
+; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
 ; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %piv)
-; CHECK: %val = load i32, i32* %ptr
   
 entry:
   br label %loop
 
 loop:
   %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
   %piv = getelementptr i32, i32* %ptr, i32 %x
   call {}* @llvm.invariant.start.p0i32(i64 4, i32* %piv)
   %val = load i32, i32* %ptr
   %x.inc = add i32 %x, %val
   br label %loop
 }
 
-; Should be able to hoist since store doesn't alias
+;; can hoist the load since the store doesn't alias -- due to
+;; the unknown instruction handling, the store ends up in a separate
+;; aliasing set
 define void @test3(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @test3(
 ; CHECK-LABEL: entry:
+; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
 ; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
-; CHECK: %val = load i32, i32* %ptr
 
 entry:
   br label %loop
 
 loop:
   %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
   call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
   %val = load i32, i32* %ptr
loop:
   store i32 0, i32* %ptr
   call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
   %val = load i32, i32* %ptr
   %x.inc = add i32 %x, %val
   br label %loop
 }
 
 ; don't try to reason about scopes
+;; FIXME: not actually testing scopes case yet
 define void @test5(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @test5(
 ; CHECK-LABEL: entry:
+; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
-; CHECK:   store i32 0, i32* %ptr
 ; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
-; CHECK: %val = load i32, i32* %ptr
 
 entry:
   br label %loop
 
 loop:
   %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
-  store i32 0, i32* %ptr
+  %p2 = getelementptr i32, i32* %ptr, i32 1
+  store i32 0, i32* %p2
   %scope = call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
   %val = load i32, i32* %ptr
 ;;  NOTE: despite being correct syntax, uncommenting this line causes
 ;;  a crash in the optimizer.  FIXME
 ;;  call void @llvm.invariant.end.p0i32({}* %scope, i64 4, i32* %ptr)
   %x.inc = add i32 %x, %val
   br label %loop
 }
 
 declare {}* @llvm.invariant.start.p0i32(i64, i32*)
 declare void @llvm.invariant.end.p0i32({}*, i64, i32*)