[AVR] Fix displacement overflow for LDDW/STDW

In some cases, the code generator attempts to generate instructions such as:

lddw r24, Y+63

which expand to:

ldd r24, Y+63
ldd r25, Y+64 # Oops! This is actually ld r25, Y in the binary

This commit limits the first offset to 62, and thus the second to 63.
It also updates some asserts in AVRExpandPseudoInsts.cpp, including for
INW and OUTW, which appear to be unused.

Author: exscape

lib/Target/AVR/AVRExpandPseudoInsts.cpp

@@ -699,7 +699,9 @@ bool AVRExpandPseudo::expand<AVR::LDDWRdPtrQ>(Block &MBB, BlockIt MBBI) {
   OpHi = AVR::LDDRdPtrQ;
   TRI->splitReg(DstReg, DstLoReg, DstHiReg);
 
-  assert(Imm <= 63 && "Offset is out of range");
+  // Since we add 1 to the Imm value for the high byte below, and 63 is the highest Imm value
+  // allowed for the instruction, 62 is the limit here.
+  assert(Imm <= 62 && "Offset is out of range");
 
   // Use a temporary register if src and dst registers are the same.
   if (DstReg == SrcReg)
@@ -1078,7 +1080,9 @@ bool AVRExpandPseudo::expand<AVR::STDWPtrQRr>(Block &MBB, BlockIt MBBI) {
   OpHi = AVR::STDPtrQRr;
   TRI->splitReg(SrcReg, SrcLoReg, SrcHiReg);
 
-  assert(Imm <= 63 && "Offset is out of range");
+  // Since we add 1 to the Imm value for the high byte below, and 63 is the highest Imm value
+  // allowed for the instruction, 62 is the limit here.
+  assert(Imm <= 62 && "Offset is out of range");
 
   auto MIBLO = buildMI(MBB, MBBI, OpLo)
     .addReg(DstReg)
@@ -1108,7 +1112,9 @@ bool AVRExpandPseudo::expand<AVR::INWRdA>(Block &MBB, BlockIt MBBI) {
   OpHi = AVR::INRdA;
   TRI->splitReg(DstReg, DstLoReg, DstHiReg);
 
-  assert(Imm <= 63 && "Address is out of range");
+  // Since we add 1 to the Imm value for the high byte below, and 63 is the highest Imm value
+  // allowed for the instruction, 62 is the limit here.
+  assert(Imm <= 62 && "Address is out of range");
 
   auto MIBLO = buildMI(MBB, MBBI, OpLo)
     .addReg(DstLoReg, RegState::Define | getDeadRegState(DstIsDead))
@@ -1136,7 +1142,9 @@ bool AVRExpandPseudo::expand<AVR::OUTWARr>(Block &MBB, BlockIt MBBI) {
   OpHi = AVR::OUTARr;
   TRI->splitReg(SrcReg, SrcLoReg, SrcHiReg);
 
-  assert(Imm <= 63 && "Address is out of range");
+  // Since we add 1 to the Imm value for the high byte below, and 63 is the highest Imm value
+  // allowed for the instruction, 62 is the limit here.
+  assert(Imm <= 62 && "Address is out of range");
 
   // 16 bit I/O writes need the high byte first
   auto MIBHI = buildMI(MBB, MBBI, OpHi)

lib/Target/AVR/AVRRegisterInfo.cpp

@@ -204,7 +204,7 @@ void AVRRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   // If the offset is too big we have to adjust and restore the frame pointer
   // to materialize a valid load/store with displacement.
   //:TODO: consider using only one adiw/sbiw chain for more than one frame index
-  if (Offset > 63) {
+  if (Offset > 62) {
     unsigned AddOpc = AVR::ADIWRdK, SubOpc = AVR::SBIWRdK;
     int AddOffset = Offset - 63 + 1;
 

test/CodeGen/AVR/std-ldd-immediate-overflow.ll

@@ -0,0 +1,18 @@
+; RUN: llc -O0 < %s -march=avr | FileCheck %s
+
+define i32 @std_ldd_overflow() {
+  %src = alloca [4 x i8]
+  %dst = alloca [4 x i8]
+  %buf = alloca [28 x i16]
+  %1 = bitcast [4 x i8]* %src to i32*
+  store i32 0, i32 *%1
+  %2 = bitcast [4 x i8]* %dst to i8*
+  %3 = bitcast [4 x i8]* %src to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i16(i8* %2, i8* %3, i16 4, i32 1, i1 false)
+; CHECK-NOT: std {{[XYZ]}}+64, {{r[0-9]+}}
+; CHECK-NOT: ldd {{r[0-9]+}}, {{[XYZ]}}+64
+
+  ret i32 0
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i16(i8* nocapture writeonly, i8* nocapture readonly, i16, i32, i1)