[llvm] [xray] Handle conditional calls when lowering patchable tail calls. (PR #89364)

[Ricky Zhou via llvm-commits]

https://github.com/rickyz updated https://github.com/llvm/llvm-project/pull/89364

>From 7d4be9f554fc6ddf6ff972d2db016915adc2cd86 Mon Sep 17 00:00:00 2001
From: Ricky Zhou <ricky at rzhou.org>
Date: Fri, 19 Apr 2024 02:44:07 -0700
Subject: [PATCH] [xray] Handle conditional calls when lowering patchable tail
 calls.

xray instruments tail call function exits by inserting a nop sled before
the tail call. When tracing is enabled, the nop sled is replaced with a
call to `__xray_FunctionTailExit()`. This currently does not work for
conditional tail calls, as the instrumentation assumes that the tail
call will be unconditional. This causes two issues:
 - `__xray_FunctionTailExit()` is inappropately called even when the
   tail call is not taken.
 - `__xray_FunctionTailExit()`'s prologue/epilogue adjusts the stack
   pointer with add/sub instructions. This clobbers condition flags,
   which can flip the condition used for the tail call, leading to
   incorrect program behavior.

Fix this by rewriting conditional calls when lowering patchable tail
calls.

With this change, a conditional patchable tail call like:
```
  je target
```

Will be lowered to:
```
  jne .fallthrough
  .p2align 1, ..
.Lxray_sled_N:
  SLED_CODE
  jmp target
.fallthrough:
```
---
 llvm/lib/Target/X86/X86MCInstLower.cpp        | 40 ++++++++++++++++---
 .../CodeGen/X86/xray-conditional-tail-call.ll | 35 ++++++++++++++++
 2 files changed, 69 insertions(+), 6 deletions(-)
 create mode 100644 llvm/test/CodeGen/X86/xray-conditional-tail-call.ll

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index e6510be6b9afd0..4681c4561ddad4 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -22,6 +22,7 @@
 #include "X86RegisterInfo.h"
 #include "X86ShuffleDecodeConstantPool.h"
 #include "X86Subtarget.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
@@ -1361,6 +1362,35 @@ void X86AsmPrinter::LowerPATCHABLE_RET(const MachineInstr &MI,
 
 void X86AsmPrinter::LowerPATCHABLE_TAIL_CALL(const MachineInstr &MI,
                                              X86MCInstLower &MCIL) {
+  MCInst TC;
+  TC.setOpcode(convertTailJumpOpcode(MI.getOperand(0).getImm()));
+  // Drop the tail jump opcode.
+  auto TCOperands = drop_begin(MI.operands());
+  bool IsConditional = TC.getOpcode() == X86::JCC_1;
+  MCSymbol *FallthroughLabel;
+  if (IsConditional) {
+    // Rewrite:
+    //   je target
+    //
+    // To:
+    //   jne .fallthrough
+    //   .p2align 1, ...
+    // .Lxray_sled_N:
+    //   SLED_CODE
+    //   jmp target
+    // .fallthrough:
+    FallthroughLabel = OutContext.createTempSymbol();
+    EmitToStreamer(
+        *OutStreamer,
+        MCInstBuilder(X86::JCC_1)
+            .addExpr(MCSymbolRefExpr::create(FallthroughLabel, OutContext))
+            .addImm(X86::GetOppositeBranchCondition(
+                static_cast<X86::CondCode>(MI.getOperand(2).getImm()))));
+    TC.setOpcode(X86::JMP_1);
+    // Drop the conditional code.
+    TCOperands = drop_end(TCOperands);
+  }
+
   NoAutoPaddingScope NoPadScope(*OutStreamer);
 
   // Like PATCHABLE_RET, we have the actual instruction in the operands to this
@@ -1382,18 +1412,16 @@ void X86AsmPrinter::LowerPATCHABLE_TAIL_CALL(const MachineInstr &MI,
   OutStreamer->emitLabel(Target);
   recordSled(CurSled, MI, SledKind::TAIL_CALL, 2);
 
-  unsigned OpCode = MI.getOperand(0).getImm();
-  OpCode = convertTailJumpOpcode(OpCode);
-  MCInst TC;
-  TC.setOpcode(OpCode);
-
   // Before emitting the instruction, add a comment to indicate that this is
   // indeed a tail call.
   OutStreamer->AddComment("TAILCALL");
-  for (auto &MO : drop_begin(MI.operands()))
+  for (auto &MO : TCOperands)
     if (auto MaybeOperand = MCIL.LowerMachineOperand(&MI, MO))
       TC.addOperand(*MaybeOperand);
   OutStreamer->emitInstruction(TC, getSubtargetInfo());
+
+  if (IsConditional)
+    OutStreamer->emitLabel(FallthroughLabel);
 }
 
 // Returns instruction preceding MBBI in MachineFunction.
diff --git a/llvm/test/CodeGen/X86/xray-conditional-tail-call.ll b/llvm/test/CodeGen/X86/xray-conditional-tail-call.ll
new file mode 100644
index 00000000000000..9ceea75ca769e0
--- /dev/null
+++ b/llvm/test/CodeGen/X86/xray-conditional-tail-call.ll
@@ -0,0 +1,35 @@
+; RUN: llc -mtriple=x86_64 < %s | FileCheck %s
+
+declare void @tail_call_target()
+
+define void @conditional_tail_call(i32 %cond) "function-instrument"="xray-always" nounwind {
+  ; CHECK-LABEL: conditional_tail_call:
+  ; CHECK-NEXT:  .Lfunc_begin0:
+  ; CHECK-NEXT:  # %bb.0:
+  ; CHECK-NEXT:    .p2align 1, 0x90
+  ; CHECK-NEXT:  .Lxray_sled_0:
+  ; CHECK-NEXT:    .ascii "\353\t"
+  ; CHECK-NEXT:    nopw 512(%rax,%rax)
+  ; CHECK-NEXT:    testl %edi, %edi
+  ; CHECK-NEXT:    je .Ltmp0
+  ; CHECK-NEXT:    .p2align 1, 0x90
+  ; CHECK-NEXT:  .Lxray_sled_1:
+  ; CHECK-NEXT:    .ascii "\353\t"
+  ; CHECK-NEXT:    nopw  512(%rax,%rax)
+  ; CHECK-NEXT:  .Ltmp1:
+  ; CHECK-NEXT:    jmp tail_call_target at PLT # TAILCALL
+  ; CHECK-NEXT:  .Ltmp0:
+  ; CHECK-NEXT:  # %bb.1:
+  ; CHECK-NEXT:   .p2align  1, 0x90
+  ; CHECK-NEXT:  .Lxray_sled_2:
+  ; CHECK-NEXT:    retq
+  ; CHECK-NEXT:    nopw %cs:512(%rax,%rax)
+  ; CHECK-NEXT:  .Lfunc_end0:
+  %cmp = icmp ne i32 %cond, 0
+  br i1 %cmp, label %docall, label %ret
+docall:
+  tail call void @tail_call_target()
+  ret void
+ret:
+  ret void
+}