[LICM] Make isGuaranteedToExecute more accurate.

Summary:
Make isGuaranteedToExecute use the
isGuaranteedToTransferExecutionToSuccessor helper, and make that helper
a bit more accurate.

There's a potential performance impact here from assuming that arbitrary
calls might not return. This probably has little impact on loads and
stores to a pointer because most things alias analysis can reason about
are dereferenceable anyway. The other impacts, like less aggressive
hoisting of sdiv by a variable and less aggressive hoisting around
volatile memory operations, are unlikely to matter for real code.

This also impacts SCEV, which uses the same helper.  It's a minor
improvement there because we can tell that, for example, memcpy always
returns normally. Strictly speaking, it's also introducing
a bug, but it's not any worse than everywhere else we assume readonly
functions terminate.

Fixes http://llvm.org/PR27857.

Reviewers: hfinkel, reames, chandlerc, sanjoy

Subscribers: broune, llvm-commits

Differential Revision: http://reviews.llvm.org/D21167

llvm-svn: 272489

llvm/lib/Analysis/ValueTracking.cpp

@@ -3444,19 +3444,45 @@ OverflowResult llvm::computeOverflowForSignedAdd(Value *LHS, Value *RHS,
 }
 
 bool llvm::isGuaranteedToTransferExecutionToSuccessor(const Instruction *I) {
-  // FIXME: This conservative implementation can be relaxed. E.g. most
-  // atomic operations are guaranteed to terminate on most platforms
-  // and most functions terminate.
+  // A memory operation returns normally if it isn't volatile. A volatile
+  // operation is allowed to trap.
+  //
+  // An atomic operation isn't guaranteed to return in a reasonable amount of
+  // time because it's possible for another thread to interfere with it for an
+  // arbitrary length of time, but programs aren't allowed to rely on that.
+  if (const LoadInst *LI = dyn_cast<LoadInst>(I))
+    return !LI->isVolatile();
+  if (const StoreInst *SI = dyn_cast<StoreInst>(I))
+    return !SI->isVolatile();
+  if (const AtomicCmpXchgInst *CXI = dyn_cast<AtomicCmpXchgInst>(I))
+    return !CXI->isVolatile();
+  if (const AtomicRMWInst *RMWI = dyn_cast<AtomicRMWInst>(I))
+    return !RMWI->isVolatile();
+  if (const MemIntrinsic *MII = dyn_cast<MemIntrinsic>(I))
+    return !MII->isVolatile();
 
-  // Calls can throw and thus not terminate, and invokes may not terminate and
-  // could throw to non-successor (see bug 24185 for details).
-  if (isa<CallInst>(I) || isa<InvokeInst>(I))
-    // However, llvm.dbg intrinsics are safe, since they're no-ops.
-    return isa<DbgInfoIntrinsic>(I);
+  // If there is no successor, then execution can't transfer to it.
+  if (const auto *CRI = dyn_cast<CleanupReturnInst>(I))
+    return !CRI->unwindsToCaller();
+  if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(I))
+    return !CatchSwitch->unwindsToCaller();
+  if (isa<ResumeInst>(I))
+    return false;
+  if (isa<ReturnInst>(I))
+    return false;
 
-  return !I->isAtomic() &&       // atomics may never succeed on some platforms
-         !isa<ResumeInst>(I) &&  // has no successors
-         !isa<ReturnInst>(I);    // has no successors
+  // Calls can throw, or contain an infinite loop, or kill the process.
+  if (CallSite CS = CallSite(const_cast<Instruction*>(I))) {
+    // Calls which don't write to arbitrary memory are safe.
+    // FIXME: Ignoring infinite loops without any side-effects is too aggressive,
+    // but it's consistent with other passes. See http://llvm.org/PR965 .
+    // FIXME: This isn't aggressive enough; a call which only writes to a
+    // global is guaranteed to return.
+    return CS.onlyReadsMemory() || CS.onlyAccessesArgMemory();
+  }
+
+  // Other instructions return normally.
+  return true;
 }
 
 bool llvm::isGuaranteedToExecuteForEveryIteration(const Instruction *I,

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -388,7 +388,7 @@ void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
   // Iterate over header and compute safety info.
   for (BasicBlock::iterator I = Header->begin(), E = Header->end();
        (I != E) && !SafetyInfo->HeaderMayThrow; ++I)
-    SafetyInfo->HeaderMayThrow |= I->mayThrow();
+    SafetyInfo->HeaderMayThrow |= !isGuaranteedToTransferExecutionToSuccessor(&*I);
 
   SafetyInfo->MayThrow = SafetyInfo->HeaderMayThrow;
   // Iterate over loop instructions and compute safety info.
@@ -397,7 +397,7 @@ void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
        (BB != BBE) && !SafetyInfo->MayThrow; ++BB)
     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
          (I != E) && !SafetyInfo->MayThrow; ++I)
-      SafetyInfo->MayThrow |= I->mayThrow();
+      SafetyInfo->MayThrow |= !isGuaranteedToTransferExecutionToSuccessor(&*I);
 
   // Compute funclet colors if we might sink/hoist in a function with a funclet
   // personality routine.

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -962,5 +962,8 @@ bool llvm::isGuaranteedToExecute(const Instruction &Inst,
   if (ExitBlocks.empty())
     return false;
 
+  // FIXME: In general, we have to prove that the loop isn't an infinite loop.
+  // See http::llvm.org/PR24078 .  (The "ExitBlocks.empty()" check above is
+  // just a special case of this.)
   return true;
 }

llvm/test/Transforms/LICM/hoist-nounwind.ll

@@ -0,0 +1,71 @@
+; RUN: opt -S -basicaa -licm < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @f() nounwind
+
+; Don't hoist load past nounwind call.
+define i32 @test1(i32* noalias nocapture readonly %a) nounwind uwtable {
+; CHECK-LABEL: @test1(
+entry:
+  br label %for.body
+
+; CHECK: tail call void @f()
+; CHECK-NEXT: load i32
+for.body:
+  %i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %x.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  tail call void @f() nounwind
+  %i1 = load i32, i32* %a, align 4
+  %add = add nsw i32 %i1, %x.05
+  %inc = add nuw nsw i32 %i.06, 1
+  %exitcond = icmp eq i32 %inc, 1000
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret i32 %add
+}
+
+; Don't hoist division past nounwind call.
+define i32 @test2(i32 %N, i32 %c) nounwind uwtable {
+; CHECK-LABEL: @test2(
+entry:
+  %cmp4 = icmp sgt i32 %N, 0
+  br i1 %cmp4, label %for.body, label %for.cond.cleanup
+
+; CHECK: tail call void @f()
+; CHECK-NEXT: sdiv i32
+for.body:
+  %i.05 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+  tail call void @f() nounwind
+  %div = sdiv i32 5, %c
+  %add = add i32 %i.05, 1
+  %inc = add i32 %add, %div
+  %cmp = icmp slt i32 %inc, %N
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:
+  ret i32 0
+}
+
+; Don't hoist load past volatile load.
+define i32 @test3(i32* noalias nocapture readonly %a, i32* %v) nounwind uwtable {
+; CHECK-LABEL: @test3(
+entry:
+  br label %for.body
+
+; CHECK: load volatile i32
+; CHECK-NEXT: load i32
+for.body:
+  %i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %x.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  %xxx = load volatile i32, i32* %v, align 4
+  %i1 = load i32, i32* %a, align 4
+  %add = add nsw i32 %i1, %x.05
+  %inc = add nuw nsw i32 %i.06, 1
+  %exitcond = icmp eq i32 %inc, 1000
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret i32 %add
+}

llvm/test/Transforms/LICM/hoisting.ll

@@ -37,16 +37,21 @@ define i32 @test2(i1 %c) {
 ; CHECK-LABEL: @test2(
 ; CHECK-NEXT: load i32, i32* @X
 ; CHECK-NEXT: %B = sdiv i32 4, %A
-	%A = load i32, i32* @X		; <i32> [#uses=2]
-	br label %Loop
+  %A = load i32, i32* @X
+  br label %Loop
+
 Loop:
-        ;; Should have hoisted this div!
-	%B = sdiv i32 4, %A		; <i32> [#uses=2]
-	call void @foo2( i32 %B )
-	br i1 %c, label %Loop, label %Out
-Out:		; preds = %Loop
-	%C = sub i32 %A, %B		; <i32> [#uses=1]
-	ret i32 %C
+  ;; Should have hoisted this div!
+  %B = sdiv i32 4, %A
+  br label %loop2
+
+loop2:
+  call void @foo2( i32 %B )
+  br i1 %c, label %Loop, label %Out
+
+Out:
+  %C = sub i32 %A, %B
+  ret i32 %C
 }
 
 

llvm/test/Transforms/LICM/preheader-safe.ll

@@ -14,6 +14,9 @@ entry:
 
 loop:                                         ; preds = %entry, %for.inc
   %div = udiv i64 %x, %y
+  br label %loop2
+
+loop2:
   call void @use_nothrow(i64 %div)
   br label %loop
 }

llvm/test/Transforms/LICM/promote-tls.ll

@@ -25,7 +25,7 @@ for.header:
   %i.02 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
   %old = load i32, i32* %addr, align 4
   ; deliberate impossible to analyze branch
-  %guard = load volatile i8*, i8** @p
+  %guard = load atomic i8*, i8** @p monotonic, align 8
   %exitcmp = icmp eq i8* %guard, null
   br i1 %exitcmp, label %for.body, label %early-exit
 

llvm/test/Transforms/LICM/scalar_promote.ll

@@ -135,7 +135,7 @@ Loop:   ; preds = %Loop, %0
   %x2 = add i32 %x, 1   ; <i32> [#uses=1]
   store i32 %x2, i32* @X
 
-        store volatile i32* @X, i32** %P2
+        store atomic i32* @X, i32** %P2 monotonic, align 8
 
   %Next = add i32 %j, 1   ; <i32> [#uses=2]
   %cond = icmp eq i32 %Next, 0    ; <i1> [#uses=1]

llvm/test/Transforms/LICM/volatile-alias.ll

@@ -9,7 +9,7 @@
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
 ; Function Attrs: nounwind uwtable
-define i32 @foo(i32* %p, i32* %q, i32 %n) #0 {
+define i32 @foo(i32* dereferenceable(4) nonnull %p, i32* %q, i32 %n) #0 {
 entry:
   %p.addr = alloca i32*, align 8
   %q.addr = alloca i32*, align 8