[llvm] [BPF] Report an error if comparison imm operand cannot fit in 32bit (PR #142989)

[via llvm-commits]

https://github.com/yonghong-song updated https://github.com/llvm/llvm-project/pull/142989

>From 0ba6601909ea5129ab05762157c4894fc5e6c2a0 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Wed, 4 Jun 2025 16:22:53 -0700
Subject: [PATCH] [BPF] Report an error if comparison imm operand cannot fit in
 32bit

Ihor Solodrai reported a case ([1]) where gcc reports an error but clang
ignores that error and proceeds to generate incorrect code. More
specifically, the problematic code looks like:
  if r1 == 0xcafefeeddeadbeef goto <label>

Here, 0xcafefeeddeadbeef needs to be encoded in a 32-bit imm field
of the insns and the 32-bit imm allows sign extenstion to 64-bit imm.
Obviously, 0xcafefeeddeadbeef cannot encode properly.

The compilation failed for gcc with the following error:
  Error: immediate out of range, shall fit in 32 bits

This patch added detection of such violation and will emit the error
error message the same as gcc.

  [1] https://lore.kernel.org/bpf/f93ce37e-e155-4165-88e2-1a3cadee7c82@linux.dev/
---
 .../BPF/MCTargetDesc/BPFMCCodeEmitter.cpp     | 19 +++++++++++++------
 llvm/test/CodeGen/BPF/warn-cmp.ll             | 10 ++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)
 create mode 100644 llvm/test/CodeGen/BPF/warn-cmp.ll

diff --git a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCCodeEmitter.cpp b/llvm/lib/Target/BPF/MCTargetDesc/BPFMCCodeEmitter.cpp
index 10a46f100bbea..077522dbd46cb 100644
--- a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCCodeEmitter.cpp
+++ b/llvm/lib/Target/BPF/MCTargetDesc/BPFMCCodeEmitter.cpp
@@ -14,6 +14,7 @@
 #include "MCTargetDesc/BPFMCTargetDesc.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCCodeEmitter.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCInst.h"
@@ -33,11 +34,12 @@ namespace {
 class BPFMCCodeEmitter : public MCCodeEmitter {
   const MCRegisterInfo &MRI;
   bool IsLittleEndian;
+  MCContext &Ctx;
 
 public:
   BPFMCCodeEmitter(const MCInstrInfo &, const MCRegisterInfo &mri,
-                   bool IsLittleEndian)
-      : MRI(mri), IsLittleEndian(IsLittleEndian) { }
+                   bool IsLittleEndian, MCContext &ctx)
+      : MRI(mri), IsLittleEndian(IsLittleEndian), Ctx(ctx) { }
   BPFMCCodeEmitter(const BPFMCCodeEmitter &) = delete;
   void operator=(const BPFMCCodeEmitter &) = delete;
   ~BPFMCCodeEmitter() override = default;
@@ -67,12 +69,12 @@ class BPFMCCodeEmitter : public MCCodeEmitter {
 
 MCCodeEmitter *llvm::createBPFMCCodeEmitter(const MCInstrInfo &MCII,
                                             MCContext &Ctx) {
-  return new BPFMCCodeEmitter(MCII, *Ctx.getRegisterInfo(), true);
+  return new BPFMCCodeEmitter(MCII, *Ctx.getRegisterInfo(), true, Ctx);
 }
 
 MCCodeEmitter *llvm::createBPFbeMCCodeEmitter(const MCInstrInfo &MCII,
                                               MCContext &Ctx) {
-  return new BPFMCCodeEmitter(MCII, *Ctx.getRegisterInfo(), false);
+  return new BPFMCCodeEmitter(MCII, *Ctx.getRegisterInfo(), false, Ctx);
 }
 
 unsigned BPFMCCodeEmitter::getMachineOpValue(const MCInst &MI,
@@ -81,8 +83,13 @@ unsigned BPFMCCodeEmitter::getMachineOpValue(const MCInst &MI,
                                              const MCSubtargetInfo &STI) const {
   if (MO.isReg())
     return MRI.getEncodingValue(MO.getReg());
-  if (MO.isImm())
-    return static_cast<unsigned>(MO.getImm());
+  if (MO.isImm()) {
+    int64_t Imm = MO.getImm();
+    if (MI.getOpcode() != BPF::LD_imm64 && (Imm < INT_MIN || Imm > UINT_MAX))
+      Ctx.reportError(MI.getLoc(),
+                      "immediate out of range, shall fit in 32 bits");
+    return static_cast<unsigned>(Imm);
+  }
 
   assert(MO.isExpr());
 
diff --git a/llvm/test/CodeGen/BPF/warn-cmp.ll b/llvm/test/CodeGen/BPF/warn-cmp.ll
new file mode 100644
index 0000000000000..a58bb58c03f3c
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/warn-cmp.ll
@@ -0,0 +1,10 @@
+; RUN: not llc -mtriple=bpfel -filetype=obj < %s 2>&1 >/dev/null | FileCheck %s
+
+; CHECK: error: immediate out of range, shall fit in 32 bits
+
+define dso_local void @test() naked {
+  tail call void asm sideeffect
+    "call 7; r1 = r0; r1 <<= 32; call 7; r1 $|= r0; if r1 == 0x1deadbeef goto +1; r0 = 0; exit;",
+    "~{r0},~{r1}"()
+  unreachable
+}