[PruneEH] A naked, noinline function can return via InlineAsm

The PruneEH pass tries to annotate functions as 'noreturn' if it doesn't
see a ReturnInst.  However, a naked function containing inline assembly
can contain control flow leaving the function.

This fixes PR23971.

llvm-svn: 240876

llvm/lib/Transforms/IPO/PruneEH.cpp

@@ -25,6 +25,7 @@
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/InlineAsm.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
@@ -97,42 +98,54 @@ bool PruneEH::runOnSCC(CallGraphSCC &SCC) {
     } else {
       bool CheckUnwind = !SCCMightUnwind && !F->doesNotThrow();
       bool CheckReturn = !SCCMightReturn && !F->doesNotReturn();
+      // Determine if we should scan for InlineAsm in a naked function as it
+      // is the only way to return without a ReturnInst.  Only do this for
+      // no-inline functions as functions which may be inlined cannot
+      // meaningfully return via assembly.
+      bool CheckReturnViaAsm = CheckReturn &&
+                               F->hasFnAttribute(Attribute::Naked) &&
+                               F->hasFnAttribute(Attribute::NoInline);
 
       if (!CheckUnwind && !CheckReturn)
         continue;
 
-      // Check to see if this function performs an unwind or calls an
-      // unwinding function.
-      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
-        if (CheckUnwind && isa<ResumeInst>(BB->getTerminator())) {
-          // Uses unwind / resume!
+      for (const BasicBlock &BB : *F) {
+        const TerminatorInst *TI = BB.getTerminator();
+        if (CheckUnwind && TI->mayThrow()) {
           SCCMightUnwind = true;
-        } else if (CheckReturn && isa<ReturnInst>(BB->getTerminator())) {
+        } else if (CheckReturn && isa<ReturnInst>(TI)) {
           SCCMightReturn = true;
         }
 
-        // Invoke instructions don't allow unwinding to continue, so we are
-        // only interested in call instructions.
-        if (CheckUnwind && !SCCMightUnwind)
-          for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-            if (CallInst *CI = dyn_cast<CallInst>(I)) {
-              if (CI->doesNotThrow()) {
-                // This call cannot throw.
-              } else if (Function *Callee = CI->getCalledFunction()) {
+        for (const Instruction &I : BB) {
+          if ((!CheckUnwind || SCCMightUnwind) &&
+              (!CheckReturnViaAsm || SCCMightReturn))
+            break;
+
+          // Check to see if this function performs an unwind or calls an
+          // unwinding function.
+          if (CheckUnwind && !SCCMightUnwind && I.mayThrow()) {
+            bool InstMightUnwind = true;
+            if (const auto *CI = dyn_cast<CallInst>(&I)) {
+              if (Function *Callee = CI->getCalledFunction()) {
                 CallGraphNode *CalleeNode = CG[Callee];
-                // If the callee is outside our current SCC then we may
-                // throw because it might.
-                if (!SCCNodes.count(CalleeNode)) {
-                  SCCMightUnwind = true;
-                  break;
-                }
-              } else {
-                // Indirect call, it might throw.
-                SCCMightUnwind = true;
-                break;
+                // If the callee is outside our current SCC then we may throw
+                // because it might.  If it is inside, do nothing.
+                if (SCCNodes.count(CalleeNode) > 0)
+                  InstMightUnwind = false;
               }
             }
-        if (SCCMightUnwind && SCCMightReturn) break;
+            SCCMightUnwind |= InstMightUnwind;
+          }
+          if (CheckReturnViaAsm && !SCCMightReturn)
+            if (auto ICS = ImmutableCallSite(&I))
+              if (const auto *IA = dyn_cast<InlineAsm>(ICS.getCalledValue()))
+                if (IA->hasSideEffects())
+                  SCCMightReturn = true;
+        }
+
+        if (SCCMightUnwind && SCCMightReturn)
+          break;
       }
     }
   }

llvm/test/Transforms/PruneEH/pr23971.ll

+; RUN: opt -S -prune-eh < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @f() #0 {
+entry:
+  call void asm sideeffect "ret\0A\09", "~{dirflag},~{fpsr},~{flags}"()
+  unreachable
+}
+
+define i32 @g() {
+entry:
+  call void @f()
+  ret i32 42
+}
+
+; CHECK-LABEL: define i32 @g()
+; CHECK: ret i32 42
+
+attributes #0 = { naked noinline }