[rust] Mark inlined calls to functions in same SCC as callee as noinline

llvm/lib/Transforms/IPO/Inliner.cpp

@@ -876,8 +876,8 @@ PreservedAnalyses InlinerPass::run(LazyCallGraph::SCC &InitialC,
       // trigger infinite inlining, much like is prevented within the inliner
       // itself by the InlineHistory above, but spread across CGSCC iterations
       // and thus hidden from the full inline history.
-      if (CG.lookupSCC(*CG.lookup(Callee)) == C &&
-          UR.InlinedInternalEdges.count({&N, C})) {
+      LazyCallGraph::SCC *CalleeSCC = CG.lookupSCC(*CG.lookup(Callee));
+      if (CalleeSCC == C && UR.InlinedInternalEdges.count({&N, C})) {
         LLVM_DEBUG(dbgs() << "Skipping inlining internal SCC edge from a node "
                              "previously split out of this SCC by inlining: "
                           << F.getName() << " -> " << Callee.getName() << "\n");
@@ -935,9 +935,24 @@ PreservedAnalyses InlinerPass::run(LazyCallGraph::SCC &InitialC,
             if (tryPromoteCall(*ICB))
               NewCallee = ICB->getCalledFunction();
           }
-          if (NewCallee)
-            if (!NewCallee->isDeclaration())
+          if (NewCallee) {
+            if (!NewCallee->isDeclaration()) {
               Calls->push({ICB, NewHistoryID});
+              // Continually inlining through an SCC can result in huge compile
+              // times and bloated code since we arbitrarily stop at some point
+              // when the inliner decides it's not profitable to inline
+              // anymore. We put a stop at the first potential attempt at
+              // inlining through an SCC by marking the call site as noinline.
+              // This doesn't apply to calls in the same SCC since if we do
+              // inline through the SCC the function will end up being
+              // self-recursive which the inliner bails out on, and inlining
+              // within an SCC is necessary for performance.
+              if (CalleeSCC != C &&
+                  CalleeSCC == CG.lookupSCC(CG.get(*NewCallee))) {
+                ICB->addFnAttr(Attribute::NoInline);
+              }
+            }
+          }
         }
       }
 

llvm/test/Transforms/Inline/mut-rec-scc-2.ll

@@ -0,0 +1,19 @@
+; RUN: opt -S -passes='inline' < %s | FileCheck %s
+
+; Make sure we don't mark calls within the same SCC as original function with noinline.
+; CHECK-NOT: noinline
+
+define void @samescc1() {
+  call void @samescc2()
+  ret void
+}
+
+define void @samescc2() {
+  call void @samescc3()
+  ret void
+}
+
+define void @samescc3() {
+  call void @samescc1()
+  ret void
+}

llvm/test/Transforms/Inline/mut-rec-scc.ll

@@ -0,0 +1,68 @@
+; RUN: opt -S -passes='cgscc(inline,instcombine)' < %s | FileCheck %s
+
+; We use call to a dummy function to avoid inlining test1 into test2 or vice
+; versa, such that we aren't left with a trivial cycle, as trivial cycles are
+; special-cased to never be inlined.
+; However, InstCombine will eliminate these calls after inlining, and thus
+; make the functions eligible for inlining in their callers.
+declare void @dummy() readnone nounwind willreturn
+
+define void @test1() {
+; CHECK-LABEL: define void @test1(
+; CHECK-NEXT:    call void @test2()
+; CHECK-NEXT:    call void @test2()
+; CHECK-NEXT:    ret void
+;
+  call void @test2()
+  call void @test2()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  ret void
+}
+
+define void @test2() {
+; CHECK-LABEL: define void @test2(
+; CHECK-NEXT:    call void @test1()
+; CHECK-NEXT:    call void @test1()
+; CHECK-NEXT:    ret void
+;
+  call void @test1()
+  call void @test1()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  call void @dummy()
+  ret void
+}
+
+; We should inline the @test2 calls and mark the inlined @test1 calls as noinline
+define void @test3() {
+; CHECK-LABEL: define void @test3(
+; CHECK-NEXT:    call void @test1() #[[NOINLINE:[0-9]+]]
+; CHECK-NEXT:    call void @test1() #[[NOINLINE]]
+; CHECK-NEXT:    call void @test1() #[[NOINLINE]]
+; CHECK-NEXT:    call void @test1() #[[NOINLINE]]
+; CHECK-NEXT:    ret void
+;
+  call void @test2()
+  call void @test2()
+  ret void
+}
+
+; CHECK: [[NOINLINE]] = { noinline }