[Clang] Emit mustprogress for infinite C++ loops

[atmnpatel]

Summary

Currently, clang does not emit the mustprogress loop or function
attribute for loops with non-zero constant conditionals. Based on recent
discussion in D86844 and D86841, it seems that it would be preferable to
emit these attributes to adhere more tightly to the C++ standard as gcc
does.

I will be pushing a patch to add -ffinite-loops and -fno-finite-loops to
clang to provide more control over this momentarily.

[atmnpatel]

while(1) case was mishandled for C++11 onwards, fixed now.

[xbolva00]

clang/lib/CodeGen/CGStmt.cpp

797

Anything non-zero?

clang/test/CodeGen/attr-mustprogress-1.cpp

118

Can you add test for eg. while (42) ?

[xbolva00]

clang/test/CodeGen/attr-mustprogress-1.cpp

32

Test for (; 42; )?

[fhahn]

I think this has been addressed by some commits a while ago. Marking as changes requested to clear up review queue. Please feel free to update the patch in case there’s anything I missed.

clang/lib/CodeGen/CGStmt.cpp

void CodeGenFunction::EmitWhileStmt(const WhileStmt &S,
   // execution of the loop body.
   llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
 
   // while(1) is common, avoid extra exit blocks.  Be sure
   // to correctly handle break/continue though.
   bool EmitBoolCondBranch = true;
   bool LoopMustProgress = false;
   if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal)) {
     if (C->isOne()) {
       EmitBoolCondBranch = false;
+      if (CWithProgress())
         FnIsMustProgress = false;
     }
   } else if (LanguageRequiresProgress())
     LoopMustProgress = true;
 
   const SourceRange &R = S.getSourceRange();
   LoopStack.push(LoopHeader.getBlock(), CGM.getContext(), CGM.getCodeGenOpts(),
                  WhileAttrs, SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()), LoopMustProgress);
void CodeGenFunction::EmitDoStmt(const DoStmt &S,
 
   // "do {} while (0)" is common in macros, avoid extra blocks.  Be sure
   // to correctly handle break/continue though.
   bool EmitBoolCondBranch = true;
   bool LoopMustProgress = false;
   if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal)) {
     if (C->isZero())
       EmitBoolCondBranch = false;
-    else if (C->isOne())
+    else if (C->isOne() && CWithProgress())
       FnIsMustProgress = false;
+    else if (CPlusPlusWithProgress())
+      LoopMustProgress = true;
   } else if (LanguageRequiresProgress())
     LoopMustProgress = true;
 
   const SourceRange &R = S.getSourceRange();
   LoopStack.push(LoopBody, CGM.getContext(), CGM.getCodeGenOpts(), DoAttrs,
                  SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()), LoopMustProgress);
 
void CodeGenFunction::EmitForStmt(const ForStmt &S,
   // If there's an increment, the continue scope will be overwritten
   // later.
   JumpDest Continue = getJumpDestInCurrentScope("for.cond");
   llvm::BasicBlock *CondBlock = Continue.getBlock();
   EmitBlock(CondBlock);
 
   bool LoopMustProgress = false;
   Expr::EvalResult Result;
+
   if (LanguageRequiresProgress()) {
-    if (!S.getCond()) {
+    if (!S.getCond() && CWithProgress())
       FnIsMustProgress = false;
-    } else if (!S.getCond()->EvaluateAsInt(Result, getContext())) {
+    else if (CPlusPlusWithProgress() ||
+             !S.getCond()->EvaluateAsInt(Result, getContext()))
       LoopMustProgress = true;
-    }
-  }
 
   const SourceRange &R = S.getSourceRange();
   LoopStack.push(CondBlock, CGM.getContext(), CGM.getCodeGenOpts(), ForAttrs,
                  SourceLocToDebugLoc(R.getBegin()),
                  SourceLocToDebugLoc(R.getEnd()), LoopMustProgress);
 
   // If the for loop doesn't have an increment we can just use the
   // condition as the continue block.  Otherwise we'll need to create
if (S.getCond()) {
 
     // C99 6.8.5p2/p4: The first substatement is executed if the expression
     // compares unequal to 0.  The condition must be a scalar type.
     llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
     llvm::MDNode *Weights = createProfileOrBranchWeightsForLoop(
         S.getCond(), getProfileCount(S.getBody()), S.getBody());
 
     if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal))
-      if (C->isOne())
+      if (C->isOne() && CWithProgress())
         FnIsMustProgress = false;
 
     Builder.CreateCondBr(BoolCondVal, ForBody, ExitBlock, Weights);
 
     if (ExitBlock != LoopExit.getBlock()) {
       EmitBlock(ExitBlock);
       EmitBranchThroughCleanup(LoopExit);
     }

clang/test/CodeGen/attr-mustprogress-1.cpp

 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-attributes
 // RUN: %clang_cc1 -std=c++11 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++14 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++17 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++20 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 
 int a = 0;
 int b = 0;
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2f0v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
-// CHECK-NOT:    br label [[FOR_COND]], !llvm.loop !{{.*}}
+// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP2:!llvm.loop !.*]]
+//
 void f0() {
   for (; ;) ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2f1v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    br i1 true, label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
-// CHECK-NEXT:    br label [[FOR_COND]]
+// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP4:!llvm.loop !.*]]
 // CHECK:       for.end:
 // CHECK-NEXT:    ret void
 //
 void f1() {
   for (; 1;)
     ;
 }
 
 // CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2f2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
 // CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
-// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP2:!llvm.loop !.*]]
+// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP5:!llvm.loop !.*]]
 // CHECK:       for.end:
 // CHECK-NEXT:    ret void
 //
 void f2() {
   for (; a == b;)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z1Fv(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    br i1 true, label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
-// CHECK-NEXT:    br label [[FOR_COND]]
+// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP6:!llvm.loop !.*]]
 // CHECK:       for.end:
 // CHECK-NEXT:    br label [[FOR_COND1:%.*]]
 // CHECK:       for.cond1:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
 // CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY2:%.*]], label [[FOR_END3:%.*]]
 // CHECK:       for.body2:
-// CHECK-NEXT:    br label [[FOR_COND1]], [[LOOP4:!llvm.loop !.*]]
+// CHECK-NEXT:    br label [[FOR_COND1]], [[LOOP7:!llvm.loop !.*]]
 // CHECK:       for.end3:
 // CHECK-NEXT:    ret void
 //
 void F() {
   for (; 1;)
     ;
   for (; a == b;)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2F2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
 // CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
-// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP5:!llvm.loop !.*]]
+// CHECK-NEXT:    br label [[FOR_COND]], [[LOOP8:!llvm.loop !.*]]
 // CHECK:       for.end:
 // CHECK-NEXT:    br label [[FOR_COND1:%.*]]
 // CHECK:       for.cond1:
 // CHECK-NEXT:    br i1 true, label [[FOR_BODY2:%.*]], label [[FOR_END3:%.*]]
 // CHECK:       for.body2:
-// CHECK-NEXT:    br label [[FOR_COND1]]
+// CHECK-NEXT:    br label [[FOR_COND1]], [[LOOP9:!llvm.loop !.*]]
 // CHECK:       for.end3:
 // CHECK-NEXT:    ret void
 //
 void F2() {
   for (; a == b;)
     ;
   for (; 1;)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2w1v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
 // CHECK:       while.body:
 // CHECK-NEXT:    br label [[WHILE_BODY]]
 //
 void w1() {
   while (1)
     ;
 }
 
 // CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2w2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[WHILE_COND:%.*]]
 // CHECK:       while.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
 // CHECK-NEXT:    br i1 [[CMP]], label [[WHILE_BODY:%.*]], label [[WHILE_END:%.*]]
 // CHECK:       while.body:
-// CHECK-NEXT:    br label [[WHILE_COND]], [[LOOP6:!llvm.loop !.*]]
+// CHECK-NEXT:    br label [[WHILE_COND]], [[LOOP10:!llvm.loop !.*]]
 // CHECK:       while.end:
 // CHECK-NEXT:    ret void
 //
 void w2() {
   while (a == b)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z1Wv(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[WHILE_COND:%.*]]
 // CHECK:       while.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
 // CHECK-NEXT:    br i1 [[CMP]], label [[WHILE_BODY:%.*]], label [[WHILE_END:%.*]]
 // CHECK:       while.body:
-// CHECK-NEXT:    br label [[WHILE_COND]], [[LOOP7:!llvm.loop !.*]]
+// CHECK-NEXT:    br label [[WHILE_COND]], [[LOOP11:!llvm.loop !.*]]
 // CHECK:       while.end:
 // CHECK-NEXT:    br label [[WHILE_BODY2:%.*]]
 // CHECK:       while.body2:
 // CHECK-NEXT:    br label [[WHILE_BODY2]]
 //
 void W() {
   while (a == b)
     ;
   while (1)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2W2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
 // CHECK:       while.body:
 // CHECK-NEXT:    br label [[WHILE_BODY]]
 //
 void W2() {
   while (1)
     ;
   while (a == b)
     ;
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2d1v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[DO_BODY:%.*]]
 // CHECK:       do.body:
 // CHECK-NEXT:    br label [[DO_COND:%.*]]
 // CHECK:       do.cond:
-// CHECK-NEXT:    br i1 true, label [[DO_BODY]], label [[DO_END:%.*]]
+// CHECK-NEXT:    br i1 true, label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP12:!llvm.loop !.*]]
 // CHECK:       do.end:
 // CHECK-NEXT:    ret void
 //
 void d1() {
   do
     ;
   while (1);
 }
 
 // CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2d2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[DO_BODY:%.*]]
 // CHECK:       do.body:
 // CHECK-NEXT:    br label [[DO_COND:%.*]]
 // CHECK:       do.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
-// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP8:!llvm.loop !.*]]
+// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP13:!llvm.loop !.*]]
 // CHECK:       do.end:
 // CHECK-NEXT:    ret void
 //
 void d2() {
   do
     ;
   while (a == b);
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z1Dv(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[DO_BODY:%.*]]
 // CHECK:       do.body:
 // CHECK-NEXT:    br label [[DO_COND:%.*]]
 // CHECK:       do.cond:
-// CHECK-NEXT:    br i1 true, label [[DO_BODY]], label [[DO_END:%.*]]
+// CHECK-NEXT:    br i1 true, label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP14:!llvm.loop !.*]]
 // CHECK:       do.end:
 // CHECK-NEXT:    br label [[DO_BODY1:%.*]]
 // CHECK:       do.body1:
 // CHECK-NEXT:    br label [[DO_COND2:%.*]]
 // CHECK:       do.cond2:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
-// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY1]], label [[DO_END3:%.*]], [[LOOP9:!llvm.loop !.*]]
+// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY1]], label [[DO_END3:%.*]], [[LOOP15:!llvm.loop !.*]]
 // CHECK:       do.end3:
 // CHECK-NEXT:    ret void
 //
 void D() {
   do
     ;
   while (1);
   do
     ;
   while (a == b);
 }
 
-// CHECK: Function Attrs: noinline nounwind optnone
+// CHECK: Function Attrs: noinline nounwind optnone mustprogress
 // CHECK-LABEL: @_Z2D2v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[DO_BODY:%.*]]
 // CHECK:       do.body:
 // CHECK-NEXT:    br label [[DO_COND:%.*]]
 // CHECK:       do.cond:
 // CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @a, align 4
 // CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* @b, align 4
 // CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
-// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP10:!llvm.loop !.*]]
+// CHECK-NEXT:    br i1 [[CMP]], label [[DO_BODY]], label [[DO_END:%.*]], [[LOOP16:!llvm.loop !.*]]
 // CHECK:       do.end:
 // CHECK-NEXT:    br label [[DO_BODY1:%.*]]
 // CHECK:       do.body1:
 // CHECK-NEXT:    br label [[DO_COND2:%.*]]
 // CHECK:       do.cond2:
-// CHECK-NEXT:    br i1 true, label [[DO_BODY1]], label [[DO_END3:%.*]]
+// CHECK-NEXT:    br i1 true, label [[DO_BODY1]], label [[DO_END3:%.*]], [[LOOP17:!llvm.loop !.*]]
 // CHECK:       do.end3:
 // CHECK-NEXT:    ret void
 //
 void D2() {
   do
     ;
   while (a == b);
   do
     ;
   while (1);
 }