[ARM][FIX] Ran out of registers due tail recursion

[dnsampaio]

Summary

• pr42062

When compiling for MinSize,
ARMTargetLowering::LowerCall decides to indirect
multiple calls to a same function. However,
it disconsiders the limitation that thumb1
indirect calls require the callee to be in a
register from r0 to r3 (llvm limiation).
If all those registers are used by arguments, the
compiler dies with "error: run out of registers
during register allocation".
This patch tells the function
IsEligibleForTailCallOptimization if we intend to
perform indirect calls, as to avoid tail call
optimization.

[dmgreen]

Nice one. Please remember to clang-format the patch.

lib/Target/ARM/ARMISelLowering.cpp

1883–1885

This looks like an unrelated change (and a little harder to parse to me, with the comment and newly between the if and the else).

1883–1885

Also, is isSibCall ever different to isTailCall? Do we need both?

test/CodeGen/ARM/pr42062.ll

8

Its probably best to test the whole output, to check it's using correct registers.

[dnsampaio]

[ARM][FIX] Ran out of registers due tail recursion

• Removed isSibCall in favor of isTailCall
• clang-format-diff
• Updated test, now checking for correct registers

[dnsampaio]

lib/Target/ARM/ARMISelLowering.cpp

1883–1885

Indeed it is not used. Will do a little of clean up as well by removing it.

[dmgreen]

I think this looks OK, but is would be good to get @efriedma's opinion.

lib/Target/ARM/ARMISelLowering.cpp

2329

Perhaps call this "IsIndirect" on this side of the call?

[efriedma]

LGTM

Eventually I think the code in IsEligibleForTailCallOptimization will disappear if we make ip an allocatable register, but this seems okay for now.

lib/Target/ARM/ARMISelLowering.h

SDValue LowerCall(TargetLowering::CallLoweringInfo &CLI,
                       SmallVectorImpl<SDValue> &InVals) const override;
 
     /// HandleByVal - Target-specific cleanup for ByVal support.
     void HandleByVal(CCState *, unsigned &, unsigned) const override;
 
     /// IsEligibleForTailCallOptimization - Check whether the call is eligible
     /// for tail call optimization. Targets which want to do tail call
     /// optimization should implement this function.
-    bool IsEligibleForTailCallOptimization(SDValue Callee,
-                                           CallingConv::ID CalleeCC,
-                                           bool isVarArg,
-                                           bool isCalleeStructRet,
-                                           bool isCallerStructRet,
-                                    const SmallVectorImpl<ISD::OutputArg> &Outs,
-                                    const SmallVectorImpl<SDValue> &OutVals,
-                                    const SmallVectorImpl<ISD::InputArg> &Ins,
-                                           SelectionDAG& DAG) const;
+    bool IsEligibleForTailCallOptimization(
+        SDValue Callee, CallingConv::ID CalleeCC, bool isVarArg,
+        bool isCalleeStructRet, bool isCallerStructRet,
+        const SmallVectorImpl<ISD::InputArg> &Ins, SelectionDAG &DAG,
+        const bool PreferIndirect) const;
 
     bool CanLowerReturn(CallingConv::ID CallConv,
                         MachineFunction &MF, bool isVarArg,
                         const SmallVectorImpl<ISD::OutputArg> &Outs,
                         LLVMContext &Context) const override;
 
     SDValue LowerReturn(SDValue Chain, CallingConv::ID CallConv, bool isVarArg,
                         const SmallVectorImpl<ISD::OutputArg> &Outs,

lib/Target/ARM/ARMISelLowering.cpp

ARMTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
   SDValue Chain                         = CLI.Chain;
   SDValue Callee                        = CLI.Callee;
   bool &isTailCall                      = CLI.IsTailCall;
   CallingConv::ID CallConv              = CLI.CallConv;
   bool doesNotRet                       = CLI.DoesNotReturn;
   bool isVarArg                         = CLI.IsVarArg;
 
   MachineFunction &MF = DAG.getMachineFunction();
-  bool isStructRet    = (Outs.empty()) ? false : Outs[0].Flags.isSRet();
-  bool isThisReturn   = false;
-  bool isSibCall      = false;
+  bool isStructRet = (Outs.empty()) ? false : Outs[0].Flags.isSRet();
+  bool isThisReturn = false;
   auto Attr = MF.getFunction().getFnAttribute("disable-tail-calls");
+  bool PreferIndirect = false;
 
   // Disable tail calls if they're not supported.
   if (!Subtarget->supportsTailCall() || Attr.getValueAsString() == "true")
     isTailCall = false;
 
+  if (isa<GlobalAddressSDNode>(Callee)) {
+    // If we're optimizing for minimum size and the function is called three or
+    // more times in this block, we can improve codesize by calling indirectly
+    // as BLXr has a 16-bit encoding.
+    auto *GV = cast<GlobalAddressSDNode>(Callee)->getGlobal();
+    auto *BB = CLI.CS.getParent();
+    PreferIndirect =
+        Subtarget->isThumb() && Subtarget->hasMinSize() &&
+        count_if(GV->users(), [&BB](const User *U) {
+          return isa<Instruction>(U) && cast<Instruction>(U)->getParent() == BB;
+        }) > 2;
+  }
   if (isTailCall) {
     // Check if it's really possible to do a tail call.
-    isTailCall = IsEligibleForTailCallOptimization(Callee, CallConv,
-                    isVarArg, isStructRet, MF.getFunction().hasStructRetAttr(),
-                                                   Outs, OutVals, Ins, DAG);
+    isTailCall = IsEligibleForTailCallOptimization(
+        Callee, CallConv, isVarArg, isStructRet,
+        MF.getFunction().hasStructRetAttr(), Outs, OutVals, Ins, DAG,
+        PreferIndirect);
     if (!isTailCall && CLI.CS && CLI.CS.isMustTailCall())
       report_fatal_error("failed to perform tail call elimination on a call "
                          "site marked musttail");
     // We don't support GuaranteedTailCallOpt for ARM, only automatically
     // detected sibcalls.
-    if (isTailCall) {
+    if (isTailCall)
       ++NumTailCalls;
-      isSibCall = true;
-    }
   }
 
   // Analyze operands of the call, assigning locations to each operand.
   SmallVector<CCValAssign, 16> ArgLocs;
   CCState CCInfo(CallConv, isVarArg, DAG.getMachineFunction(), ArgLocs,
                  *DAG.getContext());
   CCInfo.AnalyzeCallOperands(Outs, CCAssignFnForCall(CallConv, isVarArg));
 
   // Get a count of how many bytes are to be pushed on the stack.
   unsigned NumBytes = CCInfo.getNextStackOffset();
 
+  if (isTailCall) {
     // For tail calls, memory operands are available in our caller's stack.
-  if (isSibCall)
     NumBytes = 0;
-
+  } else {
     // Adjust the stack pointer for the new arguments...
     // These operations are automatically eliminated by the prolog/epilog pass
-  if (!isSibCall)
     Chain = DAG.getCALLSEQ_START(Chain, NumBytes, 0, dl);
+  }
 
   SDValue StackPtr =
       DAG.getCopyFromReg(Chain, dl, ARM::SP, getPointerTy(DAG.getDataLayout()));
 
   RegsToPassVector RegsToPass;
   SmallVector<SDValue, 8> MemOpChains;
 
   // Walk the register/memloc assignments, inserting copies/loads.  In the case
if (VA.needsCustom()) {
         SDValue AlignNode = DAG.getConstant(Flags.getByValAlign(), dl,
                                             MVT::i32);
 
         SDVTList VTs = DAG.getVTList(MVT::Other, MVT::Glue);
         SDValue Ops[] = { Chain, Dst, Src, SizeNode, AlignNode};
         MemOpChains.push_back(DAG.getNode(ARMISD::COPY_STRUCT_BYVAL, dl, VTs,
                                           Ops));
       }
-    } else if (!isSibCall) {
+    } else if (!isTailCall) {
       assert(VA.isMemLoc());
 
       MemOpChains.push_back(LowerMemOpCallTo(Chain, StackPtr, Arg,
                                              dl, DAG, VA, Flags));
     }
   }
 
   if (!MemOpChains.empty())
if (isa<GlobalAddressSDNode>(Callee)) {
       // Get the address of the callee into a register
       SDValue CPAddr = DAG.getTargetConstantPool(CPV, PtrVt, 4);
       CPAddr = DAG.getNode(ARMISD::Wrapper, dl, MVT::i32, CPAddr);
       Callee = DAG.getLoad(
           PtrVt, dl, DAG.getEntryNode(), CPAddr,
           MachinePointerInfo::getConstantPool(DAG.getMachineFunction()));
     }
   } else if (isa<GlobalAddressSDNode>(Callee)) {
-    // If we're optimizing for minimum size and the function is called three or
-    // more times in this block, we can improve codesize by calling indirectly
-    // as BLXr has a 16-bit encoding.
-    auto *GV = cast<GlobalAddressSDNode>(Callee)->getGlobal();
-    auto *BB = CLI.CS.getParent();
-    bool PreferIndirect =
-        Subtarget->isThumb() && Subtarget->hasMinSize() &&
-        count_if(GV->users(), [&BB](const User *U) {
-          return isa<Instruction>(U) && cast<Instruction>(U)->getParent() == BB;
-        }) > 2;
-
     if (!PreferIndirect) {
       isDirect = true;
       bool isDef = GV->isStrongDefinitionForLinker();
 
       // ARM call to a local ARM function is predicable.
       isLocalARMFunc = !Subtarget->isThumb() && (isDef || !ARMInterworking);
       // tBX takes a register source operand.
       if (isStub && Subtarget->isThumb1Only() && !Subtarget->hasV5TOps()) {
bool MatchingStackOffset(SDValue Arg, unsigned Offset, ISD::ArgFlagsTy Flags,
   if (!MFI.isFixedObjectIndex(FI))
     return false;
   return Offset == MFI.getObjectOffset(FI) && Bytes == MFI.getObjectSize(FI);
 }
 
 /// IsEligibleForTailCallOptimization - Check whether the call is eligible
 /// for tail call optimization. Targets which want to do tail call
 /// optimization should implement this function.
-bool
-ARMTargetLowering::IsEligibleForTailCallOptimization(SDValue Callee,
-                                                     CallingConv::ID CalleeCC,
-                                                     bool isVarArg,
-                                                     bool isCalleeStructRet,
-                                                     bool isCallerStructRet,
-                                    const SmallVectorImpl<ISD::OutputArg> &Outs,
-                                    const SmallVectorImpl<SDValue> &OutVals,
-                                    const SmallVectorImpl<ISD::InputArg> &Ins,
-                                                     SelectionDAG& DAG) const {
+bool ARMTargetLowering::IsEligibleForTailCallOptimization(
+    SDValue Callee, CallingConv::ID CalleeCC, bool isVarArg,
+    bool isCalleeStructRet, bool isCallerStructRet,
+    const SmallVectorImpl<ISD::InputArg> &Ins, SelectionDAG &DAG,
+    const bool PreferIndirect) const {
   MachineFunction &MF = DAG.getMachineFunction();
   const Function &CallerF = MF.getFunction();
   CallingConv::ID CallerCC = CallerF.getCallingConv();
 
   assert(Subtarget->supportsTailCall());
 
-  // Tail calls to function pointers cannot be optimized for Thumb1 if the args
+  // Indirect tail calls cannot be optimized for Thumb1 if the args
   // to the call take up r0-r3. The reason is that there are no legal registers
   // left to hold the pointer to the function to be called.
   if (Subtarget->isThumb1Only() && Outs.size() >= 4 &&
-      !isa<GlobalAddressSDNode>(Callee.getNode()))
-      return false;
+      (!isa<GlobalAddressSDNode>(Callee.getNode()) || PreferIndirect))
 
   // Look for obvious safe cases to perform tail call optimization that do not
   // require ABI changes. This is what gcc calls sibcall.
 
   // Exception-handling functions need a special set of instructions to indicate
   // a return to the hardware. Tail-calling another function would probably
   // break this.
   if (CallerF.hasFnAttribute("interrupt"))

test/CodeGen/ARM/pr42062.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -o - %s 2>&1 | FileCheck %s --implicit-check-not=error
+target triple = "thumbv8m.base-arm-none-eabi"
+@foo = external global i8
+declare i32 @bar(i8* nocapture, i32, i32, i8* nocapture)
+
+define void @food(i8* %a) #0 {
+; CHECK-LABEL: food:
+; CHECK:    mov [[ARG0:r[4-7]]], r0
+; CHECK-NEXT:    movs r1, #8
+; CHECK-NEXT:    movs r2, #1
+; CHECK-NEXT:    ldr [[FOO_R:r[4-7]]], [[FOO_ADDR:\..*]]
+; CHECK-NEXT:    ldr [[BAR_R:r[4-7]]], [[BAR_ADDR:\..*]]
+; CHECK-NEXT:    mov r3, [[FOO_R]]
+; CHECK-NEXT:    blx [[BAR_R]]
+; CHECK-NEXT:    movs r1, #9
+; CHECK-NEXT:    movs r2, #0
+; CHECK-NEXT:    mov r0, [[ARG0]]
+; CHECK-NEXT:    mov r3, [[FOO_R]]
+; CHECK-NEXT:    blx [[BAR_R]]
+; CHECK-NEXT:    movs r1, #7
+; CHECK-NEXT:    movs r2, #2
+; CHECK-NEXT:    mov r0, [[ARG0]]
+; CHECK-NEXT:    mov r3, [[FOO_R]]
+; CHECK-NEXT:    blx [[BAR_R]]
+; CHECK-NEXT:    pop {r4, r5, r6, pc}
+; CHECK:         [[FOO_ADDR]]:
+; CHECK-NEXT:    .long foo
+; CHECK:         [[BAR_ADDR]]:
+; CHECK-NEXT:    .long bar
+entry:
+  %0 = tail call i32 @bar(i8* %a, i32 8, i32 1, i8* nonnull @foo)
+  %1 = tail call i32 @bar(i8* %a, i32 9, i32 0, i8* nonnull @foo)
+  %2 = tail call i32 @bar(i8* %a, i32 7, i32 2, i8* nonnull @foo)
+  ret void
+}
+attributes #0 = { minsize "target-cpu"="cortex-m23" }
+