[AVR] Set R31R30 as clobbered after ADJCALLSTACKUP

[aykevl]

Summary

In most cases, using R31R30 is fine because the call (which always
precedes ADJCALLSTACKUP) will clobber R31R30 anyway. However, in some
rare cases the register allocator might insert an instruction between
the call and the ADJCALLSTACKUP instruction and expect the register
pair to be live afterwards. I think this happens as a result of
rematerialization. Therefore, to fix this, the instruction needs to have
Defs set to R31R30.

Setting the Defs field does have the effect of making the instruction
look dead, which it certainly is not. This is fixed by setting
hasSideEffects to true.

---

Note that I can only reproduce the bug on a special branch that I'm working on. Maybe it is reproduce outside that branch with some effort. However, because it looks like a potential bug (and because it fixes a TODO) I've decided to create a patch to fix this anyway.

[aykevl]

I think there is a better way to do this: by always using a frame pointer when a function needs a call frame. That would simplify the code a lot.

[aykevl]

Nevermind, that won't work. This looks like the right way forward.

I have two other patches that fix a similar bug in ADJCALLSTACKDOWN. That fix is unfortunately a lot harder.

llvm/lib/Target/AVR/AVRInstrInfo.td

 let Defs = [SP, SREG],
 Uses = [SP] in
 {
   def ADJCALLSTACKDOWN : Pseudo<(outs),
                                 (ins i16imm:$amt, i16imm:$amt2),
                                 "#ADJCALLSTACKDOWN",
                                 [(AVRcallseq_start timm:$amt, timm:$amt2)]>;
 
-  // R31R30 is used to update SP, since it is a scratch reg and this instruction
-  // is placed after the function call then R31R30 should be always free.
-  //let Defs = [R31R30],
-  //Uses = [R31R30] in
-  //:TODO: if we enable this, the pseudo is killed because it looks dead
+  // R31R30 is used to update SP. It is normally free because it is a
+  // call-clobbered register but it is necessary to set it as a def as the
+  // register allocator might use it in rare cases (for rematerialization, it
+  // seems). hasSideEffects needs to be set to true so this instruction isn't
+  // considered dead.
+  let Defs = [R31R30],
+  hasSideEffects=1 in
   def ADJCALLSTACKUP : Pseudo<(outs),
                               (ins i16imm:$amt1, i16imm:$amt2),
                               "#ADJCALLSTACKUP",
                               [(AVRcallseq_end timm:$amt1, timm:$amt2)]>;
 }
 
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