[AVR] Fix stack size in functions with a frame pointer

This patch fixes a bug in stack save/restore code. Because the frame
pointer was saved/restored manually (not by marking it as clobbered) the
StackSize variable was not updated accordingly. Most code still worked,
but code that tried to load a parameter passed on the stack did not.

This commit fixes this by marking the frame pointer as a
callee-clobbered register. This will let it be saved without any effort
in prolog/epilog code and will make sure the correct address is
calculated for loading parameters that are passed on the stack.

This approach is used by most other targets (such as X86, AArch64 and
RISC-V).

Differential Revision: https://reviews.llvm.org/D78579

llvm/lib/Target/AVR/AVRFrameLowering.cpp

@@ -65,13 +65,6 @@ void AVRFrameLowering::emitPrologue(MachineFunction &MF,
         .setMIFlag(MachineInstr::FrameSetup);
   }
 
-  // Save the frame pointer if we have one.
-  if (HasFP) {
-    BuildMI(MBB, MBBI, DL, TII.get(AVR::PUSHWRr))
-        .addReg(AVR::R29R28, RegState::Kill)
-        .setMIFlag(MachineInstr::FrameSetup);
-  }
-
   // Emit special prologue code to save R1, R0 and SREG in interrupt/signal
   // handlers before saving any other registers.
   if (AFI->isInterruptOrSignalHandler()) {
@@ -168,9 +161,6 @@ void AVRFrameLowering::emitEpilogue(MachineFunction &MF,
     BuildMI(MBB, MBBI, DL, TII.get(AVR::POPWRd), AVR::R1R0);
   }
 
-  if (hasFP(MF))
-    BuildMI(MBB, MBBI, DL, TII.get(AVR::POPWRd), AVR::R29R28);
-
   // Early exit if there is no need to restore the frame pointer.
   if (!FrameSize) {
     return;
@@ -411,8 +401,10 @@ void AVRFrameLowering::determineCalleeSaves(MachineFunction &MF,
   TargetFrameLowering::determineCalleeSaves(MF, SavedRegs, RS);
 
   // If we have a frame pointer, the Y register needs to be saved as well.
-  // We don't do that here however - the prologue and epilogue generation
-  // code will handle it specially.
+  if (hasFP(MF)) {
+    SavedRegs.set(AVR::R29);
+    SavedRegs.set(AVR::R28);
+  }
 }
 /// The frame analyzer pass.
 ///

llvm/test/CodeGen/AVR/calling-conv/c/stack.ll

@@ -11,15 +11,15 @@ define void @ret_void_args_i64_i64_i32(i64 %a, i64 %b, i32 %c) {
   ; CHECK-NEXT: in      r29, 62
 
   ; Load the top two bytes from the 32-bit int.
-  ; CHECK-NEXT: ldd     r24, Y+5
-  ; CHECK-NEXT: ldd     r25, Y+6
+  ; CHECK-NEXT: ldd     r24, Y+7
+  ; CHECK-NEXT: ldd     r25, Y+8
   ; Store the top two bytes of the 32-bit int to memory.
   ; CHECK-NEXT: sts     7, r25
   ; CHECK-NEXT: sts     6, r24
 
   ; Load the bottom two bytes from the 32-bit int.
-  ; CHECK-NEXT: ldd     r24, Y+3
-  ; CHECK-NEXT: ldd     r25, Y+4
+  ; CHECK-NEXT: ldd     r24, Y+5
+  ; CHECK-NEXT: ldd     r25, Y+6
   ; Store the bottom two bytes of the 32-bit int to memory.
   ; CHECK-NEXT: sts     5, r25
   ; CHECK-NEXT: sts     4, r24

llvm/test/CodeGen/AVR/return.ll

@@ -96,14 +96,14 @@ define i64 @return64_arg2(i64 %x, i64 %y, i64 %z) {
 ; CHECK-LABEL: return64_arg2:
 ; CHECK: push r28
 ; CHECK: push r29
-; CHECK: ldd r18, Y+3
-; CHECK: ldd r19, Y+4
-; CHECK: ldd r20, Y+5
-; CHECK: ldd r21, Y+6
-; CHECK: ldd r22, Y+7
-; CHECK: ldd r23, Y+8
-; CHECK: ldd r24, Y+9
-; CHECK: ldd r25, Y+10
+; CHECK: ldd r18, Y+5
+; CHECK: ldd r19, Y+6
+; CHECK: ldd r20, Y+7
+; CHECK: ldd r21, Y+8
+; CHECK: ldd r22, Y+9
+; CHECK: ldd r23, Y+10
+; CHECK: ldd r24, Y+11
+; CHECK: ldd r25, Y+12
 ; CHECK: pop r29
 ; CHECK: pop r28
     ret i64 %z
@@ -113,10 +113,10 @@ define i32 @return64_trunc(i32 %a, i32 %b, i32 %c, i64 %d) {
 ; CHECK-LABEL: return64_trunc:
 ; CHECK: push r28
 ; CHECK: push r29
-; CHECK: ldd r22, Y+3
-; CHECK: ldd r23, Y+4
-; CHECK: ldd r24, Y+5
-; CHECK: ldd r25, Y+6
+; CHECK: ldd r22, Y+5
+; CHECK: ldd r23, Y+6
+; CHECK: ldd r24, Y+7
+; CHECK: ldd r25, Y+8
 ; CHECK: pop r29
 ; CHECK: pop r28
   %result = trunc i64 %d to i32

llvm/test/CodeGen/AVR/varargs.ll

@@ -7,12 +7,12 @@ declare void @llvm.va_end(i8*)
 define i16 @varargs1(i8* nocapture %x, ...) {
 ; CHECK-LABEL: varargs1:
 ; CHECK: movw r20, r28
-; CHECK: subi r20, 217
+; CHECK: subi r20, 215
 ; CHECK: sbci r21, 255
 ; CHECK: movw r24, r28
 ; CHECK: adiw r24, 3
-; CHECK: ldd r22, Y+37
-; CHECK: ldd r23, Y+38
+; CHECK: ldd r22, Y+39
+; CHECK: ldd r23, Y+40
 ; CHECK: call
   %buffer = alloca [32 x i8]
   %ap = alloca i8*