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

[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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AtariDreams wrote:

I think that for now, we should just emit cld if we find inline asm. We can figure out how to parse it later. Because there are also things like possible function calls within the inline asm. This would preserve the behavior prior to this patch anyway in these cases.

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