[inliner] Apply nomerge attribute to all call sites inside inlined function.

[zequanwu]

Summary

[[clang::nomerge]] statement attribute stops optimizer from merging multiple calls into one. When inlining happends, the attribute got lost, which is not desired.

[rnk]

Let's get input from someone who has made recent inliner changes. Arthur or Alina, does this change make sense to you?

[mtrofin]

lgtm

[asbirlea]

According to: https://clang.llvm.org/docs/AttributeReference.html#nomerge, the existance of the attribute should prevent call site merging, so the change makes sense.
However, in the test example then, if there was branching and two @f calls inside @bar, it would be correct to merge the @f call sites (no attribute preventing that inside) when processing @bar. But after inlining, that's no longer allowed if the attribute is distributed to all callsites inside the function.
This is a contradiction that needs clarification from folks more familiar with the Inliner.

[zequanwu]

In D92623#2438080, @asbirlea wrote:

According to: https://clang.llvm.org/docs/AttributeReference.html#nomerge, the existance of the attribute should prevent call site merging, so the change makes sense.
However, in the test example then, if there was branching and two @f calls inside @bar, it would be correct to merge the @f call sites (no attribute preventing that inside) when processing @bar. But after inlining, that's no longer allowed if the attribute is distributed to all callsites inside the function.
This is a contradiction that needs clarification from folks more familiar with the Inliner.

Good point. I didn't consider this before.

I think I should adding a new no-merge as a function attribute which prevent all the function call from merging. Why not making nomerge as both statement attribute and function attribute? Currently, it's not allowed to do so in Attr.td.

llvm/lib/Transforms/Utils/InlineFunction.cpp

if (InlinedFunctionInfo.ContainsCalls) {
     for (Function::iterator BB = FirstNewBlock, E = Caller->end(); BB != E;
          ++BB) {
       for (auto II = BB->begin(); II != BB->end();) {
         Instruction &I = *II++;
         CallInst *CI = dyn_cast<CallInst>(&I);
         if (!CI)
           continue;
 
+        // Apply nomerge attribute to all call sites inside inlined function.
+        if (CB.hasFnAttr(Attribute::NoMerge))
+          CI->setCannotMerge();
+
         // Forward varargs from inlined call site to calls to the
         // ForwardVarArgsTo function, if requested, and to musttail calls.
         if (!VarArgsToForward.empty() &&
             ((ForwardVarArgsTo &&
               CI->getCalledFunction() == ForwardVarArgsTo) ||
              CI->isMustTailCall())) {
           // Collect attributes for non-vararg parameters.
           AttributeList Attrs = CI->getAttributes();

llvm/test/Transforms/Inline/inline_nomerge.ll

+; Check nomerge attribute at call sites applying to all call sites inside the inlined function.
+; RUN: opt < %s -O3 -S | FileCheck %s
+
+declare void @f()
+declare void @g()
+
+define void @bar() {
+entry:
+    call void @f()
+    call void @g()
+    ret void
+}
+
+define void @foo(i32 %i) {
+entry:
+  switch i32 %i, label %if.end [
+    i32 5, label %if.then
+    i32 7, label %if.then2
+  ]
+
+if.then:
+  call void @bar() #0 ; CHECK:      call void @f() #0
+                      ; CHECK-NEXT: call void @g() #0
+  br label %if.end
+
+if.then2:
+  call void @bar() #0 ; CHECK: call void @f() #0
+                      ; CHECK-NEXT: call void @g() #0
+  br label %if.end
+
+if.end:
+  call void @bar() #0 ; CHECK: call void @f() #0
+                      ; CHECK-NEXT: call void @g() #0
+  ret void
+}
+
+attributes #0 = { nomerge }