[llvm] [BPF] Report warning for some insn imm requiring valid range in inline asm (PR #142989)

[via llvm-commits]

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

>From 41107109822a47f18ea5330c111cab09adbf486b 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

Given a 64-bit imm value, converting to the proper 32-bit imm value
must satisfy the following 64-bit patterns:
  00000000 00000000 00000000 00000000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
  11111111 11111111 11111111 11111111 1xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx

So if the top 32-bits is 0 or the top 33-bits is 0x1ffffffff, then the 64-bit imm
value can be truncated into proper 32-bit imm. Otherwise, a warning
message, the same as gcc will be issued. If -Werror is enabled during
compilation, the warning will turn into an error.

  [1] https://lore.kernel.org/bpf/f93ce37e-e155-4165-88e2-1a3cadee7c82@linux.dev/
---
 .../BPF/MCTargetDesc/BPFMCCodeEmitter.cpp     | 22 ++++++++++++++-----
 llvm/test/CodeGen/BPF/warn-cmp.ll             | 15 +++++++++++++
 2 files changed, 31 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..bd9d2de58c8b2 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 +83,16 @@ 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()) {
+    uint64_t Imm = MO.getImm();
+    uint64_t High32Bits = Imm >> 32, High33Bits = Imm >> 31;
+    if (MI.getOpcode() != BPF::LD_imm64 && High32Bits != 0 &&
+        High33Bits != 0x1FFFFFFFFULL) {
+      Ctx.reportWarning(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..109d177b0fb42
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/warn-cmp.ll
@@ -0,0 +1,15 @@
+; RUN: llc -mtriple=bpfel -filetype=obj < %s 2>&1 >/dev/null | FileCheck %s
+
+; CHECK: warning: immediate out of range, shall fit in 32 bits
+define dso_local void @test_1() naked {
+  tail call void asm sideeffect
+    "r1 = 40; if r1 == 0x1deadbeef goto +0; r0 = 0; exit;", "~{r0},~{r1}"()
+  unreachable
+}
+
+; CHECK: warning: immediate out of range, shall fit in 32 bits
+define dso_local void @test_2() naked {
+  tail call void asm sideeffect
+    "r1 = 40; if r1 == 0xffffffff00000000 goto +0; r0 = 0; exit;", "~{r0},~{r1}"()
+  unreachable
+}