[llvm] [BPF] Handle unreachable with a kfunc call (PR #131731)

[via llvm-commits]

================
@@ -726,6 +733,38 @@ SDValue BPFTargetLowering::LowerATOMIC_LOAD_STORE(SDValue Op,
   return Op;
 }
 
+SDValue BPFTargetLowering::LowerTRAP(SDValue Op, SelectionDAG &DAG) const {
+  MachineFunction &MF = DAG.getMachineFunction();
+  Function &F = MF.getFunction();
+  if (F.hasFnAttribute(Attribute::Naked))
+    return Op;
+
+  TargetLowering::CallLoweringInfo CLI(DAG);
+  SmallVector<SDValue> InVals;
+  SDNode *N = Op.getNode();
+  SDLoc DL(N);
+
+  Module *M = MF.getFunction().getParent();
+  FunctionType *FT = FunctionType::get(Type::getVoidTy(M->getContext()), false);
+  Function *UnreachableHelper = M->getFunction("__unreachable_helper");
+  if (!UnreachableHelper) {
+    Function *NewF = Function::Create(FT, GlobalValue::ExternalWeakLinkage,
+                                      "__unreachable_helper", M);
+    NewF->setDSOLocal(true);
+    NewF->setCallingConv(CallingConv::C);
----------------
eddyz87 wrote:

E.g. it would be good for the following not to generate too much spills:

```llvm
$ cat test6.ll
; ModuleID = 'test6.c'
source_filename = "test6.c"
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "bpf"

define void @foo(i1 %p) {
entry:
  %a = call i32 @bar()
  %b = call i32 @bar()
  %c = call i32 @bar()
  %d = call i32 @bar()
  %e = call i32 @bar()
  %f = call i32 @bar()
  %g = call i32 @bar()
  %h = call i32 @bar()
  %k = call i32 @bar()

  br i1 %p, label %tlabel, label %flabel

flabel:
  unreachable

tlabel:
  %r1 = add i32 0,   %a
  %r2 = add i32 %r1, %b
  %r3 = add i32 %r2, %c
  %r4 = add i32 %r3, %d
  %r5 = add i32 %r4, %e
  %r6 = add i32 %r5, %f
  %r7 = add i32 %r6, %g
  %r8 = add i32 %r7, %h
  %r9 = add i32 %r8, %k
  call void @buz(i32 %r9)
  ret void
}


declare dso_local i32 @bar()
declare dso_local void @buz(i32)

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!2, !3}

!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, emissionKind: FullDebug)
!1 = !DIFile(filename: "test.c", directory: "/some/dir")
!2 = !{i32 7, !"Dwarf Version", i32 5}
!3 = !{i32 2, !"Debug Info Version", i32 3}
```
```asm
$ llc -mtriple=bpfel -mcpu=v3 < test6.ll
	.file	"test6.c"
	.text
	.globl	foo                             # -- Begin function foo
	.p2align	3
	.type	foo, at function
foo:                                    # @foo
.Lfunc_begin0:
	.cfi_startproc
# %bb.0:                                # %entry
	w7 = w1
	call bar
	w6 = w0
	call bar
	*(u32 *)(r10 - 8) = w0
	call bar
	*(u32 *)(r10 - 16) = w0
	call bar
	*(u32 *)(r10 - 24) = w0
	call bar
	*(u32 *)(r10 - 32) = w0
	call bar
	*(u32 *)(r10 - 40) = w0
	call bar
	w9 = w0
	call bar
	w8 = w0
	call bar
	w7 &= 1
	if w7 != 0 goto LBB0_2
# %bb.1:                                # %flabel
	call __unreachable_helper
LBB0_2:                                 # %tlabel
	w1 = *(u32 *)(r10 - 8)
	w6 += w1
	w1 = *(u32 *)(r10 - 16)
	w6 += w1
	w1 = *(u32 *)(r10 - 24)
	w6 += w1
	w1 = *(u32 *)(r10 - 32)
	w6 += w1
	w1 = *(u32 *)(r10 - 40)
	w6 += w1
	w6 += w9
	w6 += w8
	w6 += w0
	w1 = w6
	call buz
	exit
	...
```


https://github.com/llvm/llvm-project/pull/131731