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[BPF] Allow libcalls behind a feature gate #168442

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[BPF] Allow libcalls behind a feature gate #168442

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4 changes: 4 additions & 0 deletions llvm/lib/Target/BPF/BPF.td
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,10 @@ def MisalignedMemAccess : SubtargetFeature<"allows-misaligned-mem-access",
"AllowsMisalignedMemAccess", "true",
"Allows misaligned memory access">;

def AllowBuiltinCall : SubtargetFeature<"allow-builtin-calls",
"AllowBuiltinCalls", "true",
"Allow calls to builtin functions">;

def : Proc<"generic", []>;
def : Proc<"v1", []>;
def : Proc<"v2", []>;
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23 changes: 20 additions & 3 deletions llvm/lib/Target/BPF/BPFISelLowering.cpp
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Expand Up @@ -208,6 +208,7 @@ BPFTargetLowering::BPFTargetLowering(const TargetMachine &TM,
HasMovsx = STI.hasMovsx();

AllowsMisalignedMemAccess = STI.getAllowsMisalignedMemAccess();
AllowBuiltinCalls = STI.getAllowBuiltinCalls();
}

bool BPFTargetLowering::allowsMisalignedMemoryAccesses(EVT VT, unsigned, Align,
Expand Down Expand Up @@ -567,9 +568,10 @@ SDValue BPFTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
} else if (ExternalSymbolSDNode *E = dyn_cast<ExternalSymbolSDNode>(Callee)) {
if (StringRef(E->getSymbol()) != BPF_TRAP) {
Callee = DAG.getTargetExternalSymbol(E->getSymbol(), PtrVT, 0);
fail(CLI.DL, DAG,
Twine("A call to built-in function '" + StringRef(E->getSymbol()) +
"' is not supported."));
if (!AllowBuiltinCalls)
fail(CLI.DL, DAG,
Twine("A call to built-in function '" + StringRef(E->getSymbol()) +
"' is not supported."));
}
}

Expand Down Expand Up @@ -1196,3 +1198,18 @@ bool BPFTargetLowering::isLegalAddressingMode(const DataLayout &DL,

return true;
}

bool BPFTargetLowering::shouldSignExtendTypeInLibCall(Type *Ty,
bool IsSigned) const {
return IsSigned || Ty->isIntegerTy(32);
}

bool BPFTargetLowering::CanLowerReturn(
CallingConv::ID CallConv, MachineFunction &MF, bool IsVarArg,
const SmallVectorImpl<ISD::OutputArg> &Outs, LLVMContext &Context,
const Type *RetTy) const {
// At minimal return Outs.size() <= 1, or check valid types in CC.
SmallVector<CCValAssign, 16> RVLocs;
CCState CCInfo(CallConv, IsVarArg, MF, RVLocs, Context);
return CCInfo.CheckReturn(Outs, getHasAlu32() ? RetCC_BPF32 : RetCC_BPF64);
}
10 changes: 10 additions & 0 deletions llvm/lib/Target/BPF/BPFISelLowering.h
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Expand Up @@ -68,6 +68,8 @@ class BPFTargetLowering : public TargetLowering {
// Allows Misalignment
bool AllowsMisalignedMemAccess;

bool AllowBuiltinCalls;

SDValue LowerSDIVSREM(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerDYNAMIC_STACKALLOC(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerBR_CC(SDValue Op, SelectionDAG &DAG) const;
Expand Down Expand Up @@ -163,6 +165,14 @@ class BPFTargetLowering : public TargetLowering {
MachineBasicBlock *
EmitInstrWithCustomInserterLDimm64(MachineInstr &MI,
MachineBasicBlock *BB) const;

// Returns true if arguments should be sign-extended in lib calls.
bool shouldSignExtendTypeInLibCall(Type *Ty, bool IsSigned) const override;

bool CanLowerReturn(CallingConv::ID CallConv, MachineFunction &MF,
bool IsVarArg,
const SmallVectorImpl<ISD::OutputArg> &Outs,
LLVMContext &Context, const Type *RetTy) const override;
};
}

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1 change: 1 addition & 0 deletions llvm/lib/Target/BPF/BPFSubtarget.cpp
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Expand Up @@ -70,6 +70,7 @@ void BPFSubtarget::initializeEnvironment() {
HasLoadAcqStoreRel = false;
HasGotox = false;
AllowsMisalignedMemAccess = false;
AllowBuiltinCalls = false;
}

void BPFSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
Expand Down
3 changes: 3 additions & 0 deletions llvm/lib/Target/BPF/BPFSubtarget.h
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Expand Up @@ -70,6 +70,8 @@ class BPFSubtarget : public BPFGenSubtargetInfo {
bool HasLdsx, HasMovsx, HasBswap, HasSdivSmod, HasGotol, HasStoreImm,
HasLoadAcqStoreRel, HasGotox;

bool AllowBuiltinCalls;

std::unique_ptr<CallLowering> CallLoweringInfo;
std::unique_ptr<InstructionSelector> InstSelector;
std::unique_ptr<LegalizerInfo> Legalizer;
Expand Down Expand Up @@ -101,6 +103,7 @@ class BPFSubtarget : public BPFGenSubtargetInfo {
bool hasStoreImm() const { return HasStoreImm; }
bool hasLoadAcqStoreRel() const { return HasLoadAcqStoreRel; }
bool hasGotox() const { return HasGotox; }
bool getAllowBuiltinCalls() const { return AllowBuiltinCalls; }

bool isLittleEndian() const { return IsLittleEndian; }

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2 changes: 0 additions & 2 deletions llvm/test/CodeGen/BPF/atomic-oversize.ll
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Original file line number Diff line number Diff line change
@@ -1,6 +1,4 @@
; RUN: llc -mtriple=bpf < %s | FileCheck %s
; XFAIL: *
; Doesn't currently build, with error 'only small returns supported'.

define void @test(ptr %a) nounwind {
; CHECK-LABEL: test:
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39 changes: 39 additions & 0 deletions llvm/test/CodeGen/BPF/builtin_calls.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,39 @@
; RUN: llc -march=bpfel -mattr=+allow-builtin-calls < %s | FileCheck %s
;
; C code for this test case:
;
; long func(long a, long b) {
; long x;
; return __builtin_mul_overflow(a, b, &x);
; }


declare { i64, i1 } @llvm.smul.with.overflow.i64(i64, i64)

define noundef range(i64 0, 2) i64 @func(i64 noundef %a, i64 noundef %b) local_unnamed_addr {
entry:
%0 = tail call { i64, i1 } @llvm.smul.with.overflow.i64(i64 %a, i64 %b)
%1 = extractvalue { i64, i1 } %0, 1
%conv = zext i1 %1 to i64
ret i64 %conv
}

; CHECK-LABEL: func
; CHECK: r4 = r2
; CHECK: r2 = r1
; CHECK: r3 = r2
; CHECK: r3 s>>= 63
; CHECK: r5 = r4
; CHECK: r5 s>>= 63
; CHECK: r1 = r10
; CHECK: r1 += -16
; CHECK: call __multi3
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@yonghong-song yonghong-song Nov 19, 2025

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Could you explain in the commit/description message how do you handle __multi3 function? This is clang built-in so you will implement this function in the bpf prog?

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@LucasSte LucasSte Nov 20, 2025

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When we build the core library in Rust, which implements the primitive types and their math operations, we also build as a dependency Rust's compiler-builtin crate. This crate has the implementation of __multi3 (see https://github.com/rust-lang/compiler-builtins/blob/eba1a3f3d2824739f07fe434624b0d2fee917d89/compiler-builtins/src/int/mul.rs#L108-L110).

Once we build the target program and all the dependencies (core and compiler-builtins), we merge all LLVM modules and emit a final object.

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@LucasSte LucasSte Nov 20, 2025

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Is this message clear for the commit?

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@LucasSte LucasSte Nov 20, 2025

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I updated the commit message. Let me know if it is clear. I'm happy to re-write it.

; CHECK: r1 = *(u64 *)(r10 - 16)
; CHECK: r1 s>>= 63
; CHECK: w0 = 1
; CHECK: r2 = *(u64 *)(r10 - 8)
; CHECK: if r2 != r1 goto LBB0_2
; CHECK: # %bb.1: # %entry
; CHECK: w0 = 0
; CHECK: LBB0_2: # %entry
; CHECK: exit
2 changes: 1 addition & 1 deletion llvm/test/CodeGen/BPF/struct_ret1.ll
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@@ -1,6 +1,6 @@
; RUN: not llc -mtriple=bpf < %s 2> %t1
; RUN: FileCheck %s < %t1
; CHECK: error: <unknown>:0:0: in function bar { i64, i32 } (i32, i32, i32, i32, i32): aggregate returns are not supported
; CHECK: error: <unknown>:0:0: in function bar { i64, i32 } (i32, i32, i32, i32, i32): stack arguments are not supported

%struct.S = type { i32, i32, i32 }

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2 changes: 1 addition & 1 deletion llvm/test/CodeGen/BPF/struct_ret2.ll
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@@ -1,6 +1,6 @@
; RUN: not llc -mtriple=bpf < %s 2> %t1
; RUN: FileCheck %s < %t1
; CHECK: only small returns
; CHECK: too many arguments

; Function Attrs: nounwind uwtable
define { i64, i32 } @foo(i32 %a, i32 %b, i32 %c) #0 {
Expand Down