Assure calling cld instruction in prologue of interrupt handler function

[aaboud]

Summary

Interrupt handling function cannot assume anything about the direction flag (DF in EFLAGS register), and it must clear it using "cld" instruction in these cases:

1. The interrupt handling function uses any of the "rep" instructions.
2. Interrupt handling function calls another function.

However, to simplify the implementation we just create "cld" instruction in each prologue of interrupt handler function.

[aaboud]

Can anybody take a look on this change and approve it?
It is a fix for https://llvm.org/bugs/show_bug.cgi?id=26406

Thanks

lib/Target/X86/X86FrameLowering.cpp

if (!HasFP && NumBytes) {
       BuildCFI(MBB, MBBI, DL, MCCFIInstruction::createDefCfaOffset(
                                   nullptr, -StackSize + stackGrowth));
     }
 
     // Emit DWARF info specifying the offsets of the callee-saved registers.
     if (PushedRegs)
       emitCalleeSavedFrameMoves(MBB, MBBI, DL);
   }
+
+  if (Fn->getCallingConv() == CallingConv::X86_INTR)
+    BuildMI(MBB, MBBI, DL, TII.get(X86::CLD))
+        .setMIFlag(MachineInstr::FrameSetup);
 }
 
 bool X86FrameLowering::canUseLEAForSPInEpilogue(
     const MachineFunction &MF) const {
   // We can't use LEA instructions for adjusting the stack pointer if this is a
   // leaf function in the Win64 ABI.  Only ADD instructions may be used to
   // deallocate the stack.
   // This means that we can use LEA for SP in two situations:

test/CodeGen/X86/x86-interrupt_cld.ll

+; RUN: llc -mtriple=x86_64-unknown-unknown -mattr=+avx < %s | FileCheck %s
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Checks that interrupt handler code calls cld before calling an external
+;; function.
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+; CHECK: cld
+; CHECK: call
+
+define x86_intrcc void @foo(i8* %frame) {
+  call void @bar()
+  ret void
+}
+
+declare void @bar()
+