[llvm] [RFC][SelectionDAG] Not issue TRAP node if naked function (PR #132147)
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Thu Mar 20 11:06:40 PDT 2025
https://github.com/yonghong-song updated https://github.com/llvm/llvm-project/pull/132147
>From c75fc1ba264383df257ae49f4085c053f96f62e6 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Wed, 19 Mar 2025 14:25:16 -0700
Subject: [PATCH] [SelectionDAG] Not issue TRAP node if naked function
In [1], Nikita Popov suggested that during lowering 'unreachable' insn
should not generate extra code for naked functions, and this applies
to all architectures. Note that for naked functions, 'unreachable'
insn is necessary in IR since the basic block needs a terminator to end.
This patch checked whether a function is naked function or not.
If it is a naked function, 'unreachable' insn will not generate
ISD::TRAP.
[1] https://github.com/llvm/llvm-project/pull/131731
---
llvm/include/llvm/IR/Instructions.h | 3 +++
llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp | 16 +++-----------
llvm/lib/CodeGen/SelectionDAG/FastISel.cpp | 15 +++++--------
.../SelectionDAG/SelectionDAGBuilder.cpp | 13 ++----------
llvm/lib/IR/Instructions.cpp | 21 +++++++++++++++++++
.../naked-fn-with-frame-pointer.ll | 2 --
.../X86/naked-fn-with-unreachable-trap.ll | 11 ++++++++++
7 files changed, 45 insertions(+), 36 deletions(-)
create mode 100644 llvm/test/CodeGen/X86/naked-fn-with-unreachable-trap.ll
diff --git a/llvm/include/llvm/IR/Instructions.h b/llvm/include/llvm/IR/Instructions.h
index a1f964352207f..a8f44bef51ece 100644
--- a/llvm/include/llvm/IR/Instructions.h
+++ b/llvm/include/llvm/IR/Instructions.h
@@ -4497,6 +4497,9 @@ class UnreachableInst : public Instruction {
return isa<Instruction>(V) && classof(cast<Instruction>(V));
}
+ // Whether to do target lowering in SelectionDAG.
+ bool allowISelLowering(bool TrapUnreachable, bool NoTrapAfterNoreturn) const;
+
private:
BasicBlock *getSuccessor(unsigned idx) const {
llvm_unreachable("UnreachableInst has no successors!");
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 6014f57d439f0..a709300d5e512 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -3144,20 +3144,10 @@ bool IRTranslator::translateVAArg(const User &U, MachineIRBuilder &MIRBuilder) {
}
bool IRTranslator::translateUnreachable(const User &U, MachineIRBuilder &MIRBuilder) {
- if (!MF->getTarget().Options.TrapUnreachable)
- return true;
-
auto &UI = cast<UnreachableInst>(U);
-
- // We may be able to ignore unreachable behind a noreturn call.
- if (const CallInst *Call = dyn_cast_or_null<CallInst>(UI.getPrevNode());
- Call && Call->doesNotReturn()) {
- if (MF->getTarget().Options.NoTrapAfterNoreturn)
- return true;
- // Do not emit an additional trap instruction.
- if (Call->isNonContinuableTrap())
- return true;
- }
+ if (!UI.allowISelLowering(MF->getTarget().Options.TrapUnreachable,
+ MF->getTarget().Options.NoTrapAfterNoreturn))
+ return true;
MIRBuilder.buildTrap();
return true;
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 33dd039c4ab2a..acaa276451cfd 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1854,17 +1854,12 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
}
case Instruction::Unreachable: {
- if (TM.Options.TrapUnreachable) {
- if (TM.Options.NoTrapAfterNoreturn) {
- const auto *Call =
- dyn_cast_or_null<CallInst>(cast<Instruction>(I)->getPrevNode());
- if (Call && Call->doesNotReturn())
- return true;
- }
+ auto UI = cast<UnreachableInst>(I);
+ if (!UI->allowISelLowering(TM.Options.TrapUnreachable,
+ TM.Options.NoTrapAfterNoreturn))
+ return true;
- return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
- }
- return true;
+ return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
}
case Instruction::Alloca:
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 14bb1d943d2d6..e93dbfd02f723 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3501,19 +3501,10 @@ void SelectionDAGBuilder::visitIndirectBr(const IndirectBrInst &I) {
}
void SelectionDAGBuilder::visitUnreachable(const UnreachableInst &I) {
- if (!DAG.getTarget().Options.TrapUnreachable)
+ if (!I.allowISelLowering(DAG.getTarget().Options.TrapUnreachable,
+ DAG.getTarget().Options.NoTrapAfterNoreturn))
return;
- // We may be able to ignore unreachable behind a noreturn call.
- if (const CallInst *Call = dyn_cast_or_null<CallInst>(I.getPrevNode());
- Call && Call->doesNotReturn()) {
- if (DAG.getTarget().Options.NoTrapAfterNoreturn)
- return;
- // Do not emit an additional trap instruction.
- if (Call->isNonContinuableTrap())
- return;
- }
-
DAG.setRoot(DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, DAG.getRoot()));
}
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index d2cf0ae2c1778..83f8d3ea476d6 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -4529,6 +4529,27 @@ UnreachableInst *UnreachableInst::cloneImpl() const {
return new UnreachableInst(Context);
}
+bool UnreachableInst::allowISelLowering(bool TrapUnreachable,
+ bool NoTrapAfterNoreturn) const {
+ if (!TrapUnreachable)
+ return false;
+
+ // We may be able to ignore unreachable behind a noreturn call.
+ if (const CallInst *Call = dyn_cast_or_null<CallInst>(getPrevNode());
+ Call && Call->doesNotReturn()) {
+ if (NoTrapAfterNoreturn)
+ return false;
+ // Do not emit an additional trap instruction.
+ if (Call->isNonContinuableTrap())
+ return false;
+ }
+
+ if (getFunction()->hasFnAttribute(Attribute::Naked))
+ return false;
+
+ return true;
+}
+
FreezeInst *FreezeInst::cloneImpl() const {
return new FreezeInst(getOperand(0));
}
diff --git a/llvm/test/CodeGen/WebAssembly/naked-fn-with-frame-pointer.ll b/llvm/test/CodeGen/WebAssembly/naked-fn-with-frame-pointer.ll
index fcd42e8cbfb9f..d50cf0b201229 100644
--- a/llvm/test/CodeGen/WebAssembly/naked-fn-with-frame-pointer.ll
+++ b/llvm/test/CodeGen/WebAssembly/naked-fn-with-frame-pointer.ll
@@ -9,13 +9,11 @@ define dso_local void @naked() naked "frame-pointer"="all" {
; CHECK-32: .functype naked () -> ()
; CHECK-32-NEXT: # %bb.0:
; CHECK-32-NEXT: call main
-; CHECK-32-NEXT: unreachable
;
; CHECK-64-LABEL: naked:
; CHECK-64: .functype naked () -> ()
; CHECK-64-NEXT: # %bb.0:
; CHECK-64-NEXT: call main
-; CHECK-64-NEXT: unreachable
call void @main()
unreachable
}
diff --git a/llvm/test/CodeGen/X86/naked-fn-with-unreachable-trap.ll b/llvm/test/CodeGen/X86/naked-fn-with-unreachable-trap.ll
new file mode 100644
index 0000000000000..94274fcb1c160
--- /dev/null
+++ b/llvm/test/CodeGen/X86/naked-fn-with-unreachable-trap.ll
@@ -0,0 +1,11 @@
+; RUN: llc -o - %s -mtriple=x86_64-linux-gnu -trap-unreachable | FileCheck %s
+; RUN: llc -o - %s -mtriple=x86_64-linux-gnu -trap-unreachable -fast-isel | FileCheck %s
+
+define dso_local void @foo() #0 {
+entry:
+ tail call void asm sideeffect "movl 3,%eax", "~{dirflag},~{fpsr},~{flags}"()
+ unreachable
+}
+; CHECK-NOT: ud2
+
+attributes #0 = { naked }
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