[llvm] Resolve fixme: create cld only when needed (PR #82415)

[via llvm-commits]

https://github.com/AtariDreams created https://github.com/llvm/llvm-project/pull/82415

None

>From 56d20b5687c9db6866b51d83ee14a62007923b1b Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Tue, 20 Feb 2024 15:22:01 -0500
Subject: [PATCH] Resolve fixme: create cld only when needed

---
 llvm/lib/Target/X86/X86FrameLowering.cpp | 44 +++++++++++++++++++++---
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index be416fb0db0695..569cd3330de2f2 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -2194,13 +2194,49 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF,
   // flag (DF in EFLAGS register). Clear this flag by creating "cld" instruction
   // in each prologue of interrupt handler function.
   //
-  // FIXME: Create "cld" instruction only in these cases:
+  // Create "cld" instruction only in these cases:
   // 1. The interrupt handling function uses any of the "rep" instructions.
   // 2. Interrupt handling function calls another function.
   //
-  if (Fn.getCallingConv() == CallingConv::X86_INTR)
-    BuildMI(MBB, MBBI, DL, TII.get(X86::CLD))
-        .setMIFlag(MachineInstr::FrameSetup);
+  if (Fn.getCallingConv() == CallingConv::X86_INTR) {
+    bool NeedsCLD = false;
+
+    // Check if the function calls another function.
+    for (const BasicBlock &BB : Fn) {
+      for (const Instruction &I : BB) {
+        if (isa<CallInst>(I)) {
+          NeedsCLD = true;
+          break;
+        }
+
+        // Check for rep opcode.
+        if (auto *MI = dyn_cast<MachineInstr>(&I)) {
+          unsigned Opcode = MI->getOpcode();
+          if (Opcode == X86::REP_PREFIX || Opcode == X86::REPNE_PREFIX) {
+            NeedsCLD = true;
+            break;
+          }
+        }
+      }
+    }
+
+    // Check if the function uses any "rep" instructions.
+    if (!NeedsCLD) {
+      for (const BasicBlock &BB : Fn) {
+        for (const Instruction &I : BB) {
+          if (/* check if I is a "rep" instruction */) {
+            NeedsCLD = true;
+            break;
+          }
+        }
+      }
+    }
+
+    if (NeedsCLD) {
+      BuildMI(MBB, MBBI, DL, TII.get(X86::CLD))
+          .setMIFlag(MachineInstr::FrameSetup);
+    }
+  }
 
   // At this point we know if the function has WinCFI or not.
   MF.setHasWinCFI(HasWinCFI);