[RFC][BPF] Do atomic_fetch_*() pattern matching with memory ordering

For atomic fetch_and_*() operations, do pattern matching with memory ordering
seq_cst, acq_rel, release, acquire and monotonic (relaxed). For fetch_and_*()
operations with seq_cst/acq_rel/release/acquire ordering, atomic_fetch_*()
instructions are generated. For monotonic ordering, locked insns are generated
if return value is not used. Otherwise, atomic_fetch_*() insns are used.
The main motivation is to resolve the kernel issue [1].

The following are memory ordering are supported:
  seq_cst, acq_rel, release, acquire, relaxed
Current gcc style __sync_fetch_and_*() operations are all seq_cst.

To use explicit memory ordering, the _Atomic type is needed.  The following is
an example:

```
$ cat test.c
\#include <stdatomic.h>
void f1(_Atomic int *i) {
   (void)__c11_atomic_fetch_and(i, 10, memory_order_relaxed);
}
void f2(_Atomic int *i) {
   (void)__c11_atomic_fetch_and(i, 10, memory_order_acquire);
}
void f3(_Atomic int *i) {
   (void)__c11_atomic_fetch_and(i, 10, memory_order_seq_cst);
}
$ cat run.sh
clang  -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -O2 --target=bpf -c test.c -o test.o && llvm-objdump -d test.o
$ ./run.sh

test.o: file format elf64-bpf

Disassembly of section .text:

0000000000000000 <f1>:
       0:       b4 02 00 00 0a 00 00 00 w2 = 0xa
       1:       c3 21 00 00 50 00 00 00 lock *(u32 *)(r1 + 0x0) &= w2
       2:       95 00 00 00 00 00 00 00 exit

0000000000000018 <f2>:
       3:       b4 02 00 00 0a 00 00 00 w2 = 0xa
       4:       c3 21 00 00 51 00 00 00 w2 = atomic_fetch_and((u32 *)(r1 + 0x0), w2)
       5:       95 00 00 00 00 00 00 00 exit

0000000000000030 <f3>:
       6:       b4 02 00 00 0a 00 00 00 w2 = 0xa
       7:       c3 21 00 00 51 00 00 00 w2 = atomic_fetch_and((u32 *)(r1 + 0x0), w2)
       8:       95 00 00 00 00 00 00 00 exit
```

The following is another example where return value is used:

```
$ cat test1.c
\#include <stdatomic.h>
int f1(_Atomic int *i) {
   return __c11_atomic_fetch_and(i, 10, memory_order_relaxed);
}
int f2(_Atomic int *i) {
   return __c11_atomic_fetch_and(i, 10, memory_order_acquire);
}
int f3(_Atomic int *i) {
   return __c11_atomic_fetch_and(i, 10, memory_order_seq_cst);
}
$ cat run.sh
clang  -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -O2 --target=bpf -c test1.c -o test1.o && llvm-objdump -d test1.o
$ ./run.sh

test.o: file format elf64-bpf

Disassembly of section .text:

0000000000000000 <f1>:
       0:       b4 00 00 00 0a 00 00 00 w0 = 0xa
       1:       c3 01 00 00 51 00 00 00 w0 = atomic_fetch_and((u32 *)(r1 + 0x0), w0)
       2:       95 00 00 00 00 00 00 00 exit

0000000000000018 <f2>:
       3:       b4 00 00 00 0a 00 00 00 w0 = 0xa
       4:       c3 01 00 00 51 00 00 00 w0 = atomic_fetch_and((u32 *)(r1 + 0x0), w0)
       5:       95 00 00 00 00 00 00 00 exit

0000000000000030 <f3>:
       6:       b4 00 00 00 0a 00 00 00 w0 = 0xa
       7:       c3 01 00 00 51 00 00 00 w0 = atomic_fetch_and((u32 *)(r1 + 0x0), w0)
       8:       95 00 00 00 00 00 00 00 exit
```

You can see that for relaxed memory ordering, if return value is used, atomic_fetch_and()
insn is used. Otherwise, if return value is not used, locked insn is used.

Here is another example with global _Atomic variable:

```
$ cat test3.c
\#include <stdatomic.h>

_Atomic int i;

void f1(void) {
   (void)__c11_atomic_fetch_and(&i, 10, memory_order_relaxed);
}
void f2(void) {
   (void)__c11_atomic_fetch_and(&i, 10, memory_order_seq_cst);
}
$ cat run.sh
clang  -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -O2 --target=bpf -c test3.c -o test3.o && llvm-objdump -d test3.o
$ ./run.sh

test3.o:        file format elf64-bpf

Disassembly of section .text:

0000000000000000 <f1>:
       0:       b4 01 00 00 0a 00 00 00 w1 = 0xa
       1:       18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0x0 ll
       3:       c3 12 00 00 50 00 00 00 lock *(u32 *)(r2 + 0x0) &= w1
       4:       95 00 00 00 00 00 00 00 exit

0000000000000028 <f2>:
       5:       b4 01 00 00 0a 00 00 00 w1 = 0xa
       6:       18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0x0 ll
       8:       c3 12 00 00 51 00 00 00 w1 = atomic_fetch_and((u32 *)(r2 + 0x0), w1)
       9:       95 00 00 00 00 00 00 00 exit
```

Note that in the above compilations, '-g' is not used. The reason is due to the following IR
related to _Atomic type:
```
$clang  -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -O2 --target=bpf -g -S -emit-llvm test3.c
```
The related debug info for test3.c:
```
!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
!1 = distinct !DIGlobalVariable(name: "i", scope: !2, file: !3, line: 3, type: !16, isLocal: false, isDefinition: true)
...
!16 = !DIDerivedType(tag: DW_TAG_atomic_type, baseType: !17)
!17 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
```

If compiling test.c, the related debug info:
```
...
!19 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 3, type: !20, scopeLine: 3, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !25)
!20 = !DISubroutineType(types: !21)
!21 = !{null, !22}
!22 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !23, size: 64)
!23 = !DIDerivedType(tag: DW_TAG_atomic_type, baseType: !24)
!24 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!25 = !{!26}
!26 = !DILocalVariable(name: "i", arg: 1, scope: !19, file: !1, line: 3, type: !22)
```

All the above suggests _Atomic behaves like a modifier (e.g. const, restrict, volatile).
This seems true based on doc [1].

Without proper handling DW_TAG_atomic_type, llvm BTF generation will be incorrect since
the current implementation assumes no existence of DW_TAG_atomic_type. So we have
two choices here:
  (1). llvm bpf backend processes DW_TAG_atomic_type but ignores it in BTF encoding.
  (2). Add another type, e.g., BTF_KIND_ATOMIC to BTF. BTF_KIND_ATOMIC behaves as a
       modifier like const/volatile/restrict.

For choice (1), llvm bpf backend should skip dwarf::DW_TAG_atomic_type during
BTF generation whenever necessary.

For choice (2), BTF_KIND_ATOMIC will be added to BTF so llvm backend and kernel
needs to handle that properly. The main advantage of it probably is to maintain
this atomic type so it is also available to skeleton. But I think for skeleton
a raw type might be good enough unless user space intends to do some atomic
operation with that, which is a unlikely case.

So I choose choice (1) in this RFC implementation.

 [1] https://lore.kernel.org/bpf/[email protected]/
 [2] https://dwarfstd.org/issues/131112.1.html

Author: Yonghong Song

clang/lib/Basic/Targets/BPF.cpp

@@ -37,6 +37,7 @@ void BPFTargetInfo::getTargetDefines(const LangOptions &Opts,
   }
 
   Builder.defineMacro("__BPF_FEATURE_ADDR_SPACE_CAST");
+  Builder.defineMacro("__BPF_FEATURE_ATOMIC_MEM_ORDERING");
 
   if (CPU.empty())
     CPU = "v3";

llvm/lib/Target/BPF/BPFInstrInfo.td

@@ -864,26 +864,119 @@ class XFALU32<BPFWidthModifer SizeOp, BPFArithOp Opc, string OpcodeStr,
 
 let Constraints = "$dst = $val" in {
   let Predicates = [BPFHasALU32], DecoderNamespace = "BPFALU32" in {
-    def XFADDW32 : XFALU32<BPF_W, BPF_ADD, "u32", "add", atomic_load_add_i32>;
-    def XFANDW32 : XFALU32<BPF_W, BPF_AND, "u32", "and", atomic_load_and_i32>;
-    def XFORW32  : XFALU32<BPF_W, BPF_OR,  "u32", "or",  atomic_load_or_i32>;
-    def XFXORW32 : XFALU32<BPF_W, BPF_XOR, "u32", "xor", atomic_load_xor_i32>;
+    def XFADDW32 : XFALU32<BPF_W, BPF_ADD, "u32", "add", atomic_load_add_i32_seq_cst>;
+    def XFANDW32 : XFALU32<BPF_W, BPF_AND, "u32", "and", atomic_load_and_i32_seq_cst>;
+    def XFORW32  : XFALU32<BPF_W, BPF_OR,  "u32", "or",  atomic_load_or_i32_seq_cst>;
+    def XFXORW32 : XFALU32<BPF_W, BPF_XOR, "u32", "xor", atomic_load_xor_i32_seq_cst>;
   }
 
   let Predicates = [BPFHasALU32] in {
-    def XFADDD : XFALU64<BPF_DW, BPF_ADD, "u64", "add", atomic_load_add_i64>;
+    def XFADDD : XFALU64<BPF_DW, BPF_ADD, "u64", "add", atomic_load_add_i64_seq_cst>;
   }
-  def XFANDD : XFALU64<BPF_DW, BPF_AND, "u64", "and", atomic_load_and_i64>;
-  def XFORD  : XFALU64<BPF_DW, BPF_OR,  "u64", "or",  atomic_load_or_i64>;
-  def XFXORD : XFALU64<BPF_DW, BPF_XOR, "u64", "xor", atomic_load_xor_i64>;
+  def XFANDD : XFALU64<BPF_DW, BPF_AND, "u64", "and", atomic_load_and_i64_seq_cst>;
+  def XFORD  : XFALU64<BPF_DW, BPF_OR,  "u64", "or",  atomic_load_or_i64_seq_cst>;
+  def XFXORD : XFALU64<BPF_DW, BPF_XOR, "u64", "xor", atomic_load_xor_i64_seq_cst>;
+}
+
+let Predicates = [BPFHasALU32] in {
+    def : Pat<(atomic_load_add_i32_monotonic ADDRri:$addr, GPR32:$val),
+              (XADDW32 ADDRri:$addr, GPR32:$val)>;
+    def : Pat<(atomic_load_add_i32_acquire ADDRri:$addr, GPR32:$val),
+              (XFADDW32 ADDRri:$addr, GPR32:$val)>;
+    def : Pat<(atomic_load_add_i32_release ADDRri:$addr, GPR32:$val),
+              (XFADDW32 ADDRri:$addr, GPR32:$val)>;
+    def : Pat<(atomic_load_add_i32_acq_rel ADDRri:$addr, GPR32:$val),
+              (XFADDW32 ADDRri:$addr, GPR32:$val)>;
+
+    def : Pat<(atomic_load_add_i64_monotonic ADDRri:$addr, GPR:$val),
+              (XADDD ADDRri:$addr, GPR:$val)>;
+    def : Pat<(atomic_load_add_i64_acquire ADDRri:$addr, GPR:$val),
+              (XFADDD ADDRri:$addr, GPR:$val)>;
+    def : Pat<(atomic_load_add_i64_release ADDRri:$addr, GPR:$val),
+              (XFADDD ADDRri:$addr, GPR:$val)>;
+    def : Pat<(atomic_load_add_i64_acq_rel ADDRri:$addr, GPR:$val),
+              (XFADDD ADDRri:$addr, GPR:$val)>;
 }
 
 // atomic_load_sub can be represented as a neg followed
 // by an atomic_load_add.
-def : Pat<(atomic_load_sub_i32 ADDRri:$addr, GPR32:$val),
+// FIXME: the below can probably be simplified.
+def : Pat<(atomic_load_sub_i32_monotonic ADDRri:$addr, GPR32:$val),
+          (XADDW32 ADDRri:$addr, (NEG_32 GPR32:$val))>;
+def : Pat<(atomic_load_sub_i32_acquire ADDRri:$addr, GPR32:$val),
+          (XFADDW32 ADDRri:$addr, (NEG_32 GPR32:$val))>;
+def : Pat<(atomic_load_sub_i32_release ADDRri:$addr, GPR32:$val),
+          (XFADDW32 ADDRri:$addr, (NEG_32 GPR32:$val))>;
+def : Pat<(atomic_load_sub_i32_acq_rel ADDRri:$addr, GPR32:$val),
+          (XFADDW32 ADDRri:$addr, (NEG_32 GPR32:$val))>;
+def : Pat<(atomic_load_sub_i32_seq_cst ADDRri:$addr, GPR32:$val),
           (XFADDW32 ADDRri:$addr, (NEG_32 GPR32:$val))>;
-def : Pat<(atomic_load_sub_i64 ADDRri:$addr, GPR:$val),
+
+def : Pat<(atomic_load_sub_i64_monotonic ADDRri:$addr, GPR:$val),
+          (XADDD ADDRri:$addr, (NEG_64 GPR:$val))>;
+def : Pat<(atomic_load_sub_i64_acquire ADDRri:$addr, GPR:$val),
+          (XFADDD ADDRri:$addr, (NEG_64 GPR:$val))>;
+def : Pat<(atomic_load_sub_i64_release ADDRri:$addr, GPR:$val),
+          (XFADDD ADDRri:$addr, (NEG_64 GPR:$val))>;
+def : Pat<(atomic_load_sub_i64_acq_rel ADDRri:$addr, GPR:$val),
           (XFADDD ADDRri:$addr, (NEG_64 GPR:$val))>;
+def : Pat<(atomic_load_sub_i64_seq_cst ADDRri:$addr, GPR:$val),
+          (XFADDD ADDRri:$addr, (NEG_64 GPR:$val))>;
+
+def : Pat<(atomic_load_and_i32_monotonic ADDRri:$addr, GPR32:$val),
+          (XANDW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_and_i32_acquire ADDRri:$addr, GPR32:$val),
+          (XFANDW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_and_i32_release ADDRri:$addr, GPR32:$val),
+          (XFANDW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_and_i32_acq_rel ADDRri:$addr, GPR32:$val),
+          (XFANDW32 ADDRri:$addr, GPR32:$val)>;
+
+
+def : Pat<(atomic_load_and_i64_monotonic ADDRri:$addr, GPR:$val),
+          (XANDD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_and_i64_acquire ADDRri:$addr, GPR:$val),
+          (XFANDD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_and_i64_release ADDRri:$addr, GPR:$val),
+          (XFANDD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_and_i64_acq_rel ADDRri:$addr, GPR:$val),
+          (XFANDD ADDRri:$addr, GPR:$val)>;
+
+def : Pat<(atomic_load_or_i32_monotonic ADDRri:$addr, GPR32:$val),
+          (XORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_or_i32_acquire ADDRri:$addr, GPR32:$val),
+          (XFORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_or_i32_release ADDRri:$addr, GPR32:$val),
+          (XFORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_or_i32_acq_rel ADDRri:$addr, GPR32:$val),
+          (XFORW32 ADDRri:$addr, GPR32:$val)>;
+
+def : Pat<(atomic_load_or_i64_monotonic ADDRri:$addr, GPR:$val),
+          (XORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_or_i64_acquire ADDRri:$addr, GPR:$val),
+          (XFORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_or_i64_release ADDRri:$addr, GPR:$val),
+          (XFORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_or_i64_acq_rel ADDRri:$addr, GPR:$val),
+          (XFORD ADDRri:$addr, GPR:$val)>;
+
+def : Pat<(atomic_load_xor_i32_monotonic ADDRri:$addr, GPR32:$val),
+          (XXORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_xor_i32_acquire ADDRri:$addr, GPR32:$val),
+          (XFXORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_xor_i32_release ADDRri:$addr, GPR32:$val),
+          (XFXORW32 ADDRri:$addr, GPR32:$val)>;
+def : Pat<(atomic_load_xor_i32_acq_rel ADDRri:$addr, GPR32:$val),
+          (XFXORW32 ADDRri:$addr, GPR32:$val)>;
+
+def : Pat<(atomic_load_xor_i64_monotonic ADDRri:$addr, GPR:$val),
+          (XXORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_xor_i64_acquire ADDRri:$addr, GPR:$val),
+          (XFXORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_xor_i64_release ADDRri:$addr, GPR:$val),
+          (XFXORD ADDRri:$addr, GPR:$val)>;
+def : Pat<(atomic_load_xor_i64_acq_rel ADDRri:$addr, GPR:$val),
+          (XFXORD ADDRri:$addr, GPR:$val)>;
 
 // Atomic Exchange
 class XCHG<BPFWidthModifer SizeOp, string OpcodeStr, PatFrag OpNode>

llvm/lib/Target/BPF/BPFMIChecking.cpp

@@ -43,14 +43,14 @@ struct BPFMIPreEmitChecking : public MachineFunctionPass {
   // Initialize class variables.
   void initialize(MachineFunction &MFParm);
 
-  void processAtomicInsts();
+  bool processAtomicInsts();
 
 public:
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
     if (!skipFunction(MF.getFunction())) {
       initialize(MF);
-      processAtomicInsts();
+      return processAtomicInsts();
     }
     return false;
   }
@@ -152,22 +152,91 @@ static bool hasLiveDefs(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
   return false;
 }
 
-void BPFMIPreEmitChecking::processAtomicInsts() {
+bool BPFMIPreEmitChecking::processAtomicInsts() {
+  if (!MF->getSubtarget<BPFSubtarget>().getHasJmp32()) {
+    // Only check for cpu version 1 and 2.
+    for (MachineBasicBlock &MBB : *MF) {
+      for (MachineInstr &MI : MBB) {
+        if (MI.getOpcode() != BPF::XADDW && MI.getOpcode() != BPF::XADDD)
+          continue;
+
+        LLVM_DEBUG(MI.dump());
+        if (hasLiveDefs(MI, TRI)) {
+          DebugLoc Empty;
+          const DebugLoc &DL = MI.getDebugLoc();
+          const Function &F = MF->getFunction();
+          F.getContext().diagnose(DiagnosticInfoUnsupported{
+              F, "Invalid usage of the XADD return value", DL});
+        }
+      }
+    }
+  }
+
+  // Check return values of atomic_fetch_and_{add,and,or,xor}.
+  // If the return is not used, the atomic_fetch_and_<op> instruction
+  // is replaced with atomic_<op> instruction.
+  MachineInstr *ToErase = nullptr;
+  bool Changed = false;
+  const BPFInstrInfo *TII = MF->getSubtarget<BPFSubtarget>().getInstrInfo();
   for (MachineBasicBlock &MBB : *MF) {
     for (MachineInstr &MI : MBB) {
-      if (MI.getOpcode() != BPF::XADDW && MI.getOpcode() != BPF::XADDD)
+      if (ToErase) {
+        ToErase->eraseFromParent();
+        ToErase = nullptr;
+      }
+
+      if (MI.getOpcode() != BPF::XADDW32 && MI.getOpcode() != BPF::XADDD &&
+          MI.getOpcode() != BPF::XANDW32 && MI.getOpcode() != BPF::XANDD &&
+          MI.getOpcode() != BPF::XXORW32 && MI.getOpcode() != BPF::XXORD &&
+          MI.getOpcode() != BPF::XORW32 && MI.getOpcode() != BPF::XORD)
         continue;
 
-      LLVM_DEBUG(MI.dump());
-      if (hasLiveDefs(MI, TRI)) {
-        DebugLoc Empty;
-        const DebugLoc &DL = MI.getDebugLoc();
-        const Function &F = MF->getFunction();
-        F.getContext().diagnose(DiagnosticInfoUnsupported{
-            F, "Invalid usage of the XADD return value", DL});
+      if (!hasLiveDefs(MI, TRI))
+        continue;
+
+      LLVM_DEBUG(dbgs() << "Transforming "; MI.dump());
+      unsigned newOpcode;
+      switch (MI.getOpcode()) {
+      case BPF::XADDW32:
+        newOpcode = BPF::XFADDW32;
+        break;
+      case BPF::XADDD:
+        newOpcode = BPF::XFADDD;
+        break;
+      case BPF::XANDW32:
+        newOpcode = BPF::XFANDW32;
+        break;
+      case BPF::XANDD:
+        newOpcode = BPF::XFANDD;
+        break;
+      case BPF::XXORW32:
+        newOpcode = BPF::XFXORW32;
+        break;
+      case BPF::XXORD:
+        newOpcode = BPF::XFXORD;
+        break;
+      case BPF::XORW32:
+        newOpcode = BPF::XFORW32;
+        break;
+      case BPF::XORD:
+        newOpcode = BPF::XFORD;
+        break;
+      default:
+        llvm_unreachable("Incorrect Atomic Instruction Opcode");
       }
+
+      BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(newOpcode))
+          .add(MI.getOperand(0))
+          .add(MI.getOperand(1))
+          .add(MI.getOperand(2))
+          .add(MI.getOperand(3));
+
+      ToErase = &MI;
+      Changed = true;
     }
   }
+
+  return Changed;
 }
 
 } // namespace