[llvm] [BPF] Avoid stack argument fatal error with O0 optimization (PR #206398)

[via llvm-commits]

https://github.com/yonghong-song created https://github.com/llvm/llvm-project/pull/206398

Upstream reported a compiler fatal error with stack arguments ([1]). The source code:
```
  void f(int, int, int, int, int, int);
  int main(void)
  {
        f(0, 0, 0, 0, 0, 0);
        return 0;
  }
```
The compilation flag: `clang --target=bpf -O0 -c t.c`

The failure symptom:
```
  fatal error: error in backend: Unsupported instruction : <MCInst 338 <MCOperand Imm:-8> <MCOperand Reg:2>>
```
The failure reason is due to `BPF PreEmit Peephole Optimization`. It is supposed to convert
```
  STORE_STACK_ARG_PSEUDO -8, killed $r1
```
to
```
  STD killed $r1, $r11, -8
```

Otherwise, the `BPF Assembly Printer` pass does not understand `STORE_STACK_ARG_PSEUDO` and caused the crash.

To fix the issue, two things happened:
  1. In `BPFTargetMachine.cpp`, remove `getOptLevel() != CodeGenOptLevel::None` checking. This is redundant as the same checking (with `skipFunction())` in `BPFMIPeephole.cpp`. For `skipFunction()`, if opt level is `None`, the function will be skipped.
  2. In `BPFMIPeephole.cpp`, do `expandStackArgPseudos()` before `skipFunction()` checking. This fixed the problem.

  [1] https://github.com/llvm/llvm-project/issues/205647

>From 2fe88634e6e54e6c0c35fad5f84a9f660d6ecf37 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Sun, 28 Jun 2026 19:33:59 -0700
Subject: [PATCH] [BPF] Avoid stack argument fatal error with O0 optimization

Upstream reported a compiler fatal error with stack arguments ([1]).
The source code:
  void f(int, int, int, int, int, int);
  int main(void)
  {
        f(0, 0, 0, 0, 0, 0);
        return 0;
  }

The compilation flag: clang --target=bpf -O0 -c t.c

The failure symptom:
  fatal error: error in backend: Unsupported instruction : <MCInst 338 <MCOperand Imm:-8> <MCOperand Reg:2>>

The failure reason is due to BPF PreEmit Peephole Optimization.
It is supposed to convert
  STORE_STACK_ARG_PSEUDO -8, killed $r1
to
  STD killed $r1, $r11, -8

Otherwise, the BPF Assembly Printer pass does not understand STORE_STACK_ARG_PSEUDO
and caused the crash.

To fix the issue, two things happened:
  1. In BPFTargetMachine.cpp, remove 'getOptLevel() != CodeGenOptLevel::None' checking.
     This is redundant as the same checking (with skipFunction()) in BPFMIPeephole.cpp.
     For skipFunction(), if opt level is 'None', the function will be skipped.
  2. In BPFMIPeephole.cpp, do expandStackArgPseudos() before skipFunction() checking.
This fixed the problem.

  [1] https://github.com/llvm/llvm-project/issues/205647
---
 llvm/lib/Target/BPF/BPFMIPeephole.cpp    | 13 ++++++-------
 llvm/lib/Target/BPF/BPFTargetMachine.cpp |  5 ++---
 2 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/Target/BPF/BPFMIPeephole.cpp b/llvm/lib/Target/BPF/BPFMIPeephole.cpp
index 3e5924a6ac74c..637872ee2ebb8 100644
--- a/llvm/lib/Target/BPF/BPFMIPeephole.cpp
+++ b/llvm/lib/Target/BPF/BPFMIPeephole.cpp
@@ -329,19 +329,18 @@ struct BPFMIPreEmitPeephole : public MachineFunctionPass {
 
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
-    if (skipFunction(MF.getFunction()))
-      return false;
-
     initialize(MF);
 
-    bool Changed;
-    Changed = eliminateRedundantMov();
+    bool Changed = expandStackArgPseudos();
+    if (skipFunction(MF.getFunction()))
+      return Changed;
+
+    Changed |= eliminateRedundantMov();
     if (SupportGotol)
-      Changed = adjustBranch() || Changed;
+      Changed |= adjustBranch();
     Changed |= insertMissingCallerSavedSpills();
     Changed |= removeMayGotoZero();
     Changed |= addExitAfterUnreachable();
-    Changed |= expandStackArgPseudos();
     return Changed;
   }
 };
diff --git a/llvm/lib/Target/BPF/BPFTargetMachine.cpp b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
index ad3df2c879fe7..a1ac20b841e39 100644
--- a/llvm/lib/Target/BPF/BPFTargetMachine.cpp
+++ b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
@@ -185,9 +185,8 @@ void BPFPassConfig::addMachineSSAOptimization() {
 
 void BPFPassConfig::addPreEmitPass() {
   addPass(createBPFMIPreEmitCheckingPass());
-  if (getOptLevel() != CodeGenOptLevel::None)
-    if (!DisableMIPeephole)
-      addPass(createBPFMIPreEmitPeepholePass());
+  if (!DisableMIPeephole)
+    addPass(createBPFMIPreEmitPeepholePass());
 }
 
 bool BPFPassConfig::addIRTranslator() {