[AArch64] Fix potentially incorrect SEH stack allocs for realignments

[mstorsjo]

Summary

After a07787c9a50c046e45921dd665f5a53a752bbc31, I'm not sure if it
is even possible to trigger this though.

But before possibly removing it later, it would be good to point out
that this previously was wrong - the instruction immediate encoding
for an AND instruction is entirely irrelevant for passing the raw number
of bytes to the SEH unwind opcode.

I guess it's just pure luck that it has happened to work, whenever this
has been tested (if at all).

[efriedma]

It's complete nonsense either way; we don't know how many bytes an "and" allocates.

But yes, this should be unreachable now; realignment implies a frame pointer, and a frame pointer implies we've already set NeedsWinCFI is false. (But even before a07787c9, the amount we allocate here doesn't matter; realignment implies a frame pointer, so the setfp is just going going to overwrite the stack pointer no matter what it contains.)

[mstorsjo]

In D135805#3853678, @efriedma wrote:

(But even before a07787c9, the amount we allocate here doesn't matter; realignment implies a frame pointer, so the setfp is just going going to overwrite the stack pointer no matter what it contains.)

Doh, of course. Yeah, whatever nonsense value has been written here before hasn't really had any effect anyway.

Thus I guess it's better to abandon this and make a different patch that simply removes this case and potential other cases where we add SEH opcodes that never will be included any longer.

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

if (NeedsRealignment) {
 
       BuildMI(MBB, MBBI, DL, TII->get(AArch64::ANDXri), AArch64::SP)
           .addReg(scratchSPReg, RegState::Kill)
           .addImm(andMaskEncoded);
       AFI->setStackRealigned(true);
       if (NeedsWinCFI) {
         HasWinCFI = true;
         BuildMI(MBB, MBBI, DL, TII->get(AArch64::SEH_StackAlloc))
-            .addImm(NumBytes & andMaskEncoded)
+            .addImm(NumBytes)
             .setMIFlag(MachineInstr::FrameSetup);
       }
     }
   }
 
   // If we need a base pointer, set it up here. It's whatever the value of the
   // stack pointer is at this point. Any variable size objects will be allocated
   // after this, so we can still use the base pointer to reference locals.