[llvm] [X86] Resolve FIXME: Create cld only when needed (PR #82415)

[Craig Topper via llvm-commits]

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@@ -2194,13 +2194,35 @@ 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.
+        unsigned Opcode = I.getOpcode();
+        if (X86::REP_MOVSB_32 <= Opcode && Opcode <= X86::REP_STOSW_64) {
+          NeedsCLD = true;
+          break;
+        }
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topperc wrote:

Do we need to worry about inline assembly that contains rep instructions?

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