[PowerPC] Diagnose musttail instead of crash inside backend

musttail does not often possible to generate on PPC targets as when calling
to a function defined in another model, PPC needs to restore the TOC
pointer. To restore the TOC pointer, compiler needs to emit a nop
after the function call to let linker generate codes to restore
TOC pointer. Tail call does not generate expected call sequence for this
case.

To avoid the crash inside the compiler backend, a diagnosis is added
in the frontend and in the backend, PPC will change the musttail to
tail.

Fixes #63214

Author: Chen Zheng

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3841,6 +3841,11 @@ def note_cannot_use_trivial_abi_reason : Note<
   "it is polymorphic|"
   "it has a base of a non-trivial class type|it has a virtual base|"
   "it has a __weak field|it has a field of a non-trivial class type}1">;
+def warn_ppc_musttail_maybe_ignored: Warning<
+  "'musttail' attribute may be ignored on ppc targets">,
+  InGroup<IgnoredAttributes>;
+def err_aix_musttail_unsupported: Error<
+  "'musttail' attribute is not supported on AIX">;
 
 // Availability attribute
 def warn_availability_unknown_platform : Warning<

clang/lib/CodeGen/CGCall.cpp

@@ -26,6 +26,7 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/Basic/CodeGenOptions.h"
+#include "clang/Basic/DiagnosticSema.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/CodeGen/CGFunctionInfo.h"
 #include "clang/CodeGen/SwiftCallingConv.h"
@@ -5751,8 +5752,15 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
   if (llvm::CallInst *Call = dyn_cast<llvm::CallInst>(CI)) {
     if (TargetDecl && TargetDecl->hasAttr<NotTailCalledAttr>())
       Call->setTailCallKind(llvm::CallInst::TCK_NoTail);
-    else if (IsMustTail)
+    else if (IsMustTail) {
+      if (getTarget().getTriple().isPPC()) {
+        if (getTarget().getTriple().isOSAIX())
+          CGM.getDiags().Report(Loc, diag::err_aix_musttail_unsupported);
+        else
+          CGM.getDiags().Report(Loc, diag::warn_ppc_musttail_maybe_ignored);
+      }
       Call->setTailCallKind(llvm::CallInst::TCK_MustTail);
+    }
   }
 
   // Add metadata for calls to MSAllocator functions

clang/test/SemaCXX/attr-musttail-ppc.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 %s -triple powerpc64-ibm-aix-xcoff -o /dev/null -emit-llvm -verify=aix
+// RUN: %clang_cc1 %s -triple powerpc-ibm-aix-xcoff -o /dev/null -emit-llvm -verify=aix
+// RUN: %clang_cc1 %s -triple powerpc64-unknown-linux-gnu -o /dev/null -emit-llvm -verify=linux
+// RUN: %clang_cc1 %s -triple powerpc-unknown-linux-gnu -o /dev/null -emit-llvm -verify=linux
+// RUN: %clang_cc1 %s -triple powerpc64le-unknown-linux-gnu -o /dev/null -emit-llvm -verify=linux
+
+int Func();
+int Func1() {
+  // linux-warning@+2 {{'musttail' attribute may be ignored on ppc targets}}
+  // aix-error@+1 {{'musttail' attribute is not supported on AIX}}
+  [[clang::musttail]] return Func();
+}

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -146,6 +146,10 @@ static cl::opt<unsigned> PPCAIXTLSModelOptUseIEForLDLimit(
     cl::desc("Set inclusive limit count of TLS local-dynamic access(es) in a "
              "function to use initial-exec"));
 
+static cl::opt<bool> AbortOnImpossibleMusttailCall(
+    "ppc-abort-on-impossible-musttailcall", cl::init(false), cl::Hidden,
+    cl::desc("Abort if any call marked as musttail is impossible."));
+
 STATISTIC(NumTailCalls, "Number of tail calls");
 STATISTIC(NumSiblingCalls, "Number of sibling calls");
 STATISTIC(ShufflesHandledWithVPERM,
@@ -5945,9 +5949,14 @@ PPCTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     }
   }
 
-  if (!isTailCall && CB && CB->isMustTailCall())
-    report_fatal_error("failed to perform tail call elimination on a call "
-                       "site marked musttail");
+  if (!isTailCall && CB && CB->isMustTailCall()) {
+    if (AbortOnImpossibleMusttailCall)
+      report_fatal_error("failed to perform tail call elimination on a call "
+                         "site marked musttail");
+    else
+      cast<CallInst>(const_cast<CallBase *>(CB))
+          ->setTailCallKind(llvm::CallInst::TCK_Tail);
+  }
 
   // When long calls (i.e. indirect calls) are always used, calls are always
   // made via function pointer. If we have a function name, first translate it

llvm/test/CodeGen/PowerPC/musttail-call.ll

@@ -0,0 +1,32 @@
+; RUN: not --crash llc -mtriple=powerpc64-ibm-aix-xcoff %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=true | \
+; RUN:   FileCheck %s --check-prefix=CRASH
+; RUN: not --crash llc -mtriple=powerpc-ibm-aix-xcoff %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=true | \
+; RUN:   FileCheck %s --check-prefix=CRASH
+; RUN: not --crash llc -mtriple=powerpc64-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=true | \
+; RUN:   FileCheck %s --check-prefix=CRASH
+; RUN: not --crash llc -mtriple=powerpc-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=true | \
+; RUN:   FileCheck %s --check-prefix=CRASH
+; RUN: not --crash llc -mtriple=powerpc64le-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=true | \
+; RUN:   FileCheck %s --check-prefix=CRASH
+; RUN: llc -mtriple=powerpc64-ibm-aix-xcoff %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=false | \
+; RUN:   FileCheck %s --check-prefix=NOCRASH
+; RUN: llc -mtriple=powerpc-ibm-aix-xcoff %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=false | \
+; RUN:   FileCheck %s --check-prefix=NOCRASH
+; RUN: llc -mtriple=powerpc64-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=false | \
+; RUN:   FileCheck %s --check-prefix=NOCRASH
+; RUN: llc -mtriple=powerpc-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=false | \
+; RUN:   FileCheck %s --check-prefix=NOCRASH
+; RUN: llc -mtriple=powerpc64le-unknown-linux-gnu %s -o - 2>&1 -ppc-abort-on-impossible-musttailcall=false | \
+; RUN:   FileCheck %s --check-prefix=NOCRASH
+
+; CRASH: LLVM ERROR: failed to perform tail call elimination on a call site marked musttail
+; NOCRASH-NOT: LLVM ERROR: failed to perform tail call elimination on a call site marked musttail
+; NOCRASH-LABEL: caller
+; NOCRASH:    bl {{callee|.callee}}
+
+
+declare i32 @callee()
+define i32 @caller() {
+  %res = musttail call i32 @callee()
+  ret i32 %res
+}