[JumpThreading] make jump threading respect convergent annotation.

[wengxt]

Summary

JumpThreading shouldn't duplicate a convergent call, because that would move a convergent call into a control-inequivalent location. For example,

if (cond) {
  ...
} else {
  ...
}
convergent_call();
if (cond) {
  ...
} else {
  ...
}

should not be optimized to

if (cond) {
  ...
  convergent_call();
  ...
} else {
  ...
  convergent_call();
  ...
}

Test Plan: test/Transforms/JumpThreading/basic.ll

Patch by Xuetian Weng.

[jingyue]

test/Transforms/JumpThreading/basic.ll

516

s/CHECK/CHECK-LABEL

[wengxt]

address jingyue's comment

include/llvm/IR/Instructions.h

public:
   }
 
   /// \brief Determine if the call cannot be duplicated.
   bool cannotDuplicate() const {return hasFnAttr(Attribute::NoDuplicate); }
   void setCannotDuplicate() {
     addAttribute(AttributeSet::FunctionIndex, Attribute::NoDuplicate);
   }
 
+  /// \brief Determine if the call is convergent
+  bool isConvergent() const { return hasFnAttr(Attribute::Convergent); }
+  void setConvergent() {
+    addAttribute(AttributeSet::FunctionIndex, Attribute::Convergent);
+  }
+
   /// \brief Determine if the call returns a structure through first
   /// pointer argument.
   bool hasStructRetAttr() const {
     // Be friendly and also check the callee.
     return paramHasAttr(1, Attribute::StructRet);
   }
 
   /// \brief Determine if any call argument is an aggregate passed by value.

lib/Transforms/Scalar/JumpThreading.cpp

for (; !isa<TerminatorInst>(I); ++I) {
     // All other instructions count for at least one unit.
     ++Size;
 
     // Calls are more expensive.  If they are non-intrinsic calls, we model them
     // as having cost of 4.  If they are a non-vector intrinsic, we model them
     // as having cost of 2 total, and if they are a vector intrinsic, we model
     // them as having cost 1.
     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
-      if (CI->cannotDuplicate())
+      if (CI->cannotDuplicate() || CI->isConvergent())
         // Blocks with NoDuplicate are modelled as having infinite cost, so they
         // are never duplicated.
         return ~0U;
       else if (!isa<IntrinsicInst>(CI))
         Size += 3;
       else if (!CI->getType()->isVectorTy())
         Size += 1;
     }

test/Transforms/JumpThreading/basic.ll

l4:
   ret void
 
 l5:
   call void @k()
   ret void
 ; CHECK: }
 }
 
+; CHECK: define void @h_con(i32 %p) {
+define void @h_con(i32 %p) {
+  %x = icmp ult i32 %p, 5
+  br i1 %x, label %l1, label %l2
+
+l1:
+  call void @j()
+  br label %l3
+
+l2:
+  call void @k()
+  br label %l3
+
+l3:
+; CHECK: call void @g() [[CON:#[0-9]+]]
+; CHECK-NOT: call void @g() [[CON]]
+  call void @g() convergent
+  %y = icmp ult i32 %p, 5
+  br i1 %y, label %l4, label %l5
+
+l4:
+  call void @j()
+  ret void
+
+l5:
+  call void @k()
+  ret void
+; CHECK: }
+}
+
+
 ; CHECK: attributes [[NOD]] = { noduplicate }
+; CHECK: attributes [[CON]] = { convergent }