[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 ↗

(On Diff #161100)

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

[anna]

LGTM.

llvm/trunk/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;

llvm/trunk/test/Transforms/LICM/invariant.start.ll

 ; RUN: opt -licm -basicaa -licm-n2-threshold=0 < %s -S | FileCheck %s
 ; RUN: opt -licm -basicaa -licm-n2-threshold=200 < %s -S | FileCheck %s --check-prefix=ALIAS-N2
 ; RUN: opt -aa-pipeline=basic-aa -licm-n2-threshold=0 -passes='require<aa>,require<targetir>,require<scalar-evolution>,require<opt-remark-emit>,loop(licm)' < %s -S | FileCheck %s
 ; RUN: opt -aa-pipeline=basic-aa -licm-n2-threshold=200 -passes='require<aa>,require<targetir>,require<scalar-evolution>,require<opt-remark-emit>,loop(licm)' < %s -S | FileCheck %s --check-prefix=ALIAS-N2
 
 ; TODO: By default (without the -licm-n2-threshold value), we should be able to hoist both load and 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
 
 ; ALIAS-N2-LABEL: @test1(
 ; ALIAS-N2-LABEL: entry:
 ; ALIAS-N2:         %val = load i32, i32* %ptr
 ; ALIAS-N2-LABEL: loop:
 ; ALIAS-N2: call {}* @llvm.invariant.start.p0i32(i64 4, 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: By default, despite the loop varying invariant.start, we should be
+;; despite the loop varying invariant.start, we should be
 ;; able to 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
 
 ; ALIAS-N2-LABEL: @test2(
 ; ALIAS-N2-LABEL: entry:
 ; ALIAS-N2:         %val = load i32, i32* %ptr
 ; ALIAS-N2-LABEL: loop:
 ; ALIAS-N2:         call {}* @llvm.invariant.start.p0i32(i64 4, i32* %piv)
 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
 }
 
-; By default, should be able to hoist since store doesn't alias
 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
 
 ; ALIAS-N2-LABEL: @test3(
 ; ALIAS-N2-LABEL: entry:
 ; ALIAS-N2:         %val = load i32, i32* %ptr
 ; ALIAS-N2-LABEL: loop:
 ; ALIAS-N2:         call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
 entry:
   br label %loop