[LICM] Hoist an invariant_start out of loops if there are no stores executed before it

Once the invariant_start is reached, we know that no instruction *after* it can modify the memory. So, if we can prove the location isn't read *between entry into the loop and the execution of the invariant_start*, we can execute the invariant_start before entering the loop.

Differential Revision: https://reviews.llvm.org/D51181



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340617 91177308-0d34-0410-b5e6-96231b3b80d8

Author: preames

lib/Transforms/Scalar/LICM.cpp

@@ -525,7 +525,9 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
       }
 
       using namespace PatternMatch;
-      if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>()) &&
+      if (((I.use_empty() &&
+            match(&I, m_Intrinsic<Intrinsic::invariant_start>())) ||
+           match(&I, m_Intrinsic<Intrinsic::experimental_guard>())) &&
           IsMustExecute && IsMemoryNotModified &&
           CurLoop->hasLoopInvariantOperands(&I)) {
         hoist(I, DT, CurLoop, SafetyInfo, ORE);

test/Transforms/LICM/invariant.start.ll

@@ -3,19 +3,18 @@
 ; 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: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
 ; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
-; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, 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)
+; ALIAS-N2: %val = load i32, i32* %ptr
+; ALIAS-N2-LABEL: loop:
 
 entry:
   br label %loop
@@ -57,15 +56,15 @@ loop:
 define void @test3(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @test3(
 ; CHECK-LABEL: entry:
+; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
 ; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
-; CHECK: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
 
 ; ALIAS-N2-LABEL: @test3(
 ; ALIAS-N2-LABEL: entry:
-; ALIAS-N2:         %val = load i32, i32* %ptr
+; ALIAS-N2: call {}* @llvm.invariant.start.p0i32(i64 4, i32* %ptr)
+; 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