[AVR] Fix parsing & emitting relative jumps (llvm#106722)

Ever since 6859685 (or, precisely,
84428da) relative jumps emitted by the
AVR codegen are off by two bytes - this pull request fixes it.

## Abstract

As compared to absolute jumps, relative jumps - such as rjmp, rcall or
brsh - have an implied `pc+2` behavior; that is, `jmp 100` is `pc =
100`, but `rjmp 100` gets understood as `pc = pc + 100 + 2`.

This is not reflected in the AVR codegen:

https://github.com/llvm/llvm-project/blob/f95026dbf66e353128a3a3d7b55f3e52d5985535/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.cpp#L89

... which always emits relative jumps that are two bytes too far - or
rather it _would_ emit such jumps if not for this check:

https://github.com/llvm/llvm-project/blob/f95026dbf66e353128a3a3d7b55f3e52d5985535/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.cpp#L517

... which causes most of the relative jumps to be actually resolved
late, by the linker, which applies the offsetting logic on its own,
hiding the issue within LLVM.

[Some time
ago](llvm@697a162)
we've had a similar "jumps are off" problem that got solved by touching
`shouldForceRelocation()`, but I think that has worked only by accident.
It's exploited the fact that absolute vs relative jumps in the parsed
assembly can be distinguished through a "side channel" check relying on
the existence of labels (i.e. absolute jumps happen to named labels, but
relative jumps are anonymous, so to say). This was an alright idea back
then, but it got broken by 6859685.

I propose a different approach:
- when emitting relative jumps, offset them by `-2` (well, `-1`,
strictly speaking, because those instructions rely on right-shifted
offset),
- when parsing relative jumps, treat `.` as `+2` and read `rjmp .+1234`
as `rjmp (1234 + 2)`.

This approach seems to be sound and now we generate the same assembly as
avr-gcc, which can be confirmed with:

```cpp
// avr-gcc test.c -O3 && avr-objdump -d a.out

int main() {
    asm(
"      foo:\n\t"
"        rjmp  .+2\n\t"
"        rjmp  .-2\n\t"
"        rjmp  foo\n\t"
"        rjmp  .+8\n\t"
"        rjmp  end\n\t"
"        rjmp  .+0\n\t"
"      end:\n\t"
"        rjmp .-4\n\t"
"        rjmp .-6\n\t"
"      x:\n\t"
"        rjmp x\n\t"
"        .short 0xc00f\n\t"
);
}
```

avr-gcc is also how I got the opcodes for all new tests like `inst-brbc.s`, so we should be good.

(cherry picked from commit 86a60e7)

llvm/lib/Target/AVR/AsmParser/AVRAsmParser.cpp

@@ -72,7 +72,7 @@ class AVRAsmParser : public MCTargetAsmParser {
   int parseRegisterName();
   int parseRegister(bool RestoreOnFailure = false);
   bool tryParseRegisterOperand(OperandVector &Operands);
-  bool tryParseExpression(OperandVector &Operands);
+  bool tryParseExpression(OperandVector &Operands, int64_t offset);
   bool tryParseRelocExpression(OperandVector &Operands);
   void eatComma();
 
@@ -418,7 +418,7 @@ bool AVRAsmParser::tryParseRegisterOperand(OperandVector &Operands) {
   return false;
 }
 
-bool AVRAsmParser::tryParseExpression(OperandVector &Operands) {
+bool AVRAsmParser::tryParseExpression(OperandVector &Operands, int64_t offset) {
   SMLoc S = Parser.getTok().getLoc();
 
   if (!tryParseRelocExpression(Operands))
@@ -437,6 +437,11 @@ bool AVRAsmParser::tryParseExpression(OperandVector &Operands) {
   if (getParser().parseExpression(Expression))
     return true;
 
+  if (offset) {
+    Expression = MCBinaryExpr::createAdd(
+        Expression, MCConstantExpr::create(offset, getContext()), getContext());
+  }
+
   SMLoc E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
   Operands.push_back(AVROperand::CreateImm(Expression, S, E));
   return false;
@@ -529,8 +534,9 @@ bool AVRAsmParser::parseOperand(OperandVector &Operands, bool maybeReg) {
     [[fallthrough]];
   case AsmToken::LParen:
   case AsmToken::Integer:
+    return tryParseExpression(Operands, 0);
   case AsmToken::Dot:
-    return tryParseExpression(Operands);
+    return tryParseExpression(Operands, 2);
   case AsmToken::Plus:
   case AsmToken::Minus: {
     // If the sign preceeds a number, parse the number,
@@ -540,7 +546,7 @@ bool AVRAsmParser::parseOperand(OperandVector &Operands, bool maybeReg) {
     case AsmToken::BigNum:
     case AsmToken::Identifier:
     case AsmToken::Real:
-      if (!tryParseExpression(Operands))
+      if (!tryParseExpression(Operands, 0))
         return false;
       break;
     default:
@@ -643,6 +649,7 @@ bool AVRAsmParser::ParseInstruction(ParseInstructionInfo &Info,
     // These specific operands should be treated as addresses/symbols/labels,
     // other than registers.
     bool maybeReg = true;
+
     if (OperandNum == 1) {
       std::array<StringRef, 8> Insts = {"lds", "adiw", "sbiw", "ldi"};
       for (auto Inst : Insts) {

llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.cpp

@@ -94,6 +94,9 @@ static void adjustRelativeBranch(unsigned Size, const MCFixup &Fixup,
 
   // Rightshifts the value by one.
   AVR::fixups::adjustBranchTarget(Value);
+
+  // Jumps are relative to the current instruction.
+  Value -= 1;
 }
 
 /// 22-bit absolute fixup.
@@ -513,15 +516,10 @@ bool AVRAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
   switch ((unsigned)Fixup.getKind()) {
   default:
     return Fixup.getKind() >= FirstLiteralRelocationKind;
-  // Fixups which should always be recorded as relocations.
   case AVR::fixup_7_pcrel:
   case AVR::fixup_13_pcrel:
-    // Do not force relocation for PC relative branch like 'rjmp .',
-    // 'rcall . - off' and 'breq . + off'.
-    if (const auto *SymA = Target.getSymA())
-      if (SymA->getSymbol().getName().size() == 0)
-        return false;
-    [[fallthrough]];
+    // Always resolve relocations for PC-relative branches
+    return false;
   case AVR::fixup_call:
     return true;
   }

llvm/test/CodeGen/AVR/jmp.ll

@@ -0,0 +1,25 @@
+; RUN: llc -filetype=obj -mtriple=avr < %s | llvm-objdump -dr --no-show-raw-insn - | FileCheck %s
+
+define i8 @foo(i8 %a) {
+bb0:
+  %0 = tail call i8 @bar(i8 %a)
+  %1 = icmp eq i8 %0, 123
+  br i1 %1, label %bb1, label %bb2
+
+bb1:
+  ret i8 100
+
+bb2:
+  ret i8 200
+}
+
+declare i8 @bar(i8);
+
+; CHECK: rcall   .-2
+; CHECK-NEXT: 00000000: R_AVR_13_PCREL bar
+; CHECK-NEXT: cpi     r24, 0x7b
+; CHECK-NEXT: brne    .+4
+; CHECK-NEXT: ldi     r24, 0x64
+; CHECK-NEXT: ret
+; CHECK-NEXT: ldi     r24, 0xc8
+; CHECK-NEXT: ret

llvm/test/MC/AVR/inst-brbc.s

@@ -3,22 +3,23 @@
 ; RUN:     | llvm-objdump -d - | FileCheck --check-prefix=INST %s
 
 foo:
-
   brbc 3, .+8
   brbc 0, .-16
   .short 0xf759
   .short 0xf752
   .short 0xf74c
   .short 0xf4c7
 
-; CHECK: brvc .Ltmp0+8              ; encoding: [0bAAAAA011,0b111101AA]
-; CHECK:                            ; fixup A - offset: 0, value: .Ltmp0+8, kind: fixup_7_pcrel
-; CHECK: brcc .Ltmp1-16             ; encoding: [0bAAAAA000,0b111101AA]
-; CHECK:                            ; fixup A - offset: 0, value: .Ltmp1-16, kind: fixup_7_pcrel
+; CHECK: brvc (.Ltmp0+8)+2   ; encoding: [0bAAAAA011,0b111101AA]
+; CHECK-NEXT:                ; fixup A - offset: 0, value: (.Ltmp0+8)+2, kind: fixup_7_pcrel
+;
+; CHECK: brcc (.Ltmp1-16)+2  ; encoding: [0bAAAAA000,0b111101AA]
+; CHECK-NEXT:                ; fixup A - offset: 0, value: (.Ltmp1-16)+2, kind: fixup_7_pcrel
 
-; INST: 23 f4   brvc .+8
-; INST: c0 f7   brsh .-16
-; INST: 59 f7   brne .-42
-; INST: 52 f7   brpl .-44
-; INST: 4c f7   brge .-46
-; INST: c7 f4   brid .+48
+; INST-LABEL: <foo>:
+; INST-NEXT: 23 f4   brvc .+8
+; INST-NEXT: c0 f7   brsh .-16
+; INST-NEXT: 59 f7   brne .-42
+; INST-NEXT: 52 f7   brpl .-44
+; INST-NEXT: 4c f7   brge .-46
+; INST-NEXT: c7 f4   brid .+48

llvm/test/MC/AVR/inst-brbs.s

@@ -3,22 +3,22 @@
 ; RUN:     | llvm-objdump -d - | FileCheck --check-prefix=INST %s
 
 foo:
-
   brbs 3, .+8
   brbs 0, .-12
   .short 0xf359
   .short 0xf352
   .short 0xf34c
   .short 0xf077
 
-; CHECK: brvs .Ltmp0+8              ; encoding: [0bAAAAA011,0b111100AA]
-; CHECK:                            ; fixup A - offset: 0, value: .Ltmp0+8, kind: fixup_7_pcrel
-; CHECK: brcs .Ltmp1-12             ; encoding: [0bAAAAA000,0b111100AA]
-; CHECK:                            ; fixup A - offset: 0, value: .Ltmp1-12, kind: fixup_7_pcrel
+; CHECK: brvs (.Ltmp0+8)+2   ; encoding: [0bAAAAA011,0b111100AA]
+; CHECK-NEXT:                ; fixup A - offset: 0, value: (.Ltmp0+8)+2, kind: fixup_7_pcrel
+; CHECK: brcs (.Ltmp1-12)+2  ; encoding: [0bAAAAA000,0b111100AA]
+; CHECK-NEXT:                ; fixup A - offset: 0, value: (.Ltmp1-12)+2, kind: fixup_7_pcrel
 
-; INST: 23 f0   brvs .+8
-; INST: d0 f3   brlo .-12
-; INST: 59 f3   breq .-42
-; INST: 52 f3   brmi .-44
-; INST: 4c f3   brlt .-46
-; INST: 77 f0   brie .+28
+; INST-LABEL: <foo>:
+; INST-NEXT: 23 f0   brvs .+8
+; INST-NEXT: d0 f3   brlo .-12
+; INST-NEXT: 59 f3   breq .-42
+; INST-NEXT: 52 f3   brmi .-44
+; INST-NEXT: 4c f3   brlt .-46
+; INST-NEXT: 77 f0   brie .+28

llvm/test/MC/AVR/inst-brcc.s

@@ -0,0 +1,28 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brcc .+66
+  brcc .-22
+  brbc 0, .+66
+  brbc 0, bar
+
+bar:
+
+; CHECK: brcc (.Ltmp0+66)+2  ; encoding: [0bAAAAA000,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0+66)+2, kind: fixup_7_pcrel
+; CHECK: brcc (.Ltmp1-22)+2  ; encoding: [0bAAAAA000,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1-22)+2, kind: fixup_7_pcrel
+; CHECK: brcc (.Ltmp2+66)+2  ; encoding: [0bAAAAA000,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp2+66)+2, kind: fixup_7_pcrel
+; CHECK: brcc bar            ; encoding: [0bAAAAA000,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: 08 f5      brsh .+66
+; INST-NEXT: a8 f7      brsh .-22
+; INST-NEXT: 08 f5      brsh .+66
+; INST-NEXT: 00 f4      brsh .+0

llvm/test/MC/AVR/inst-brcs.s

@@ -0,0 +1,28 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brcs .+8
+  brcs .+4
+  brbs 0, .+8
+  brbs 0, bar
+
+bar:
+
+; CHECK: brcs (.Ltmp0+8)+2  ; encoding: [0bAAAAA000,0b111100AA]
+; CHECK-NEXT:               ;   fixup A - offset: 0, value: (.Ltmp0+8)+2, kind: fixup_7_pcrel
+; CHECK: brcs (.Ltmp1+4)+2  ; encoding: [0bAAAAA000,0b111100AA]
+; CHECK-NEXT:               ;   fixup A - offset: 0, value: (.Ltmp1+4)+2, kind: fixup_7_pcrel
+; CHECK: brcs (.Ltmp2+8)+2  ; encoding: [0bAAAAA000,0b111100AA]
+; CHECK-NEXT:               ;   fixup A - offset: 0, value: (.Ltmp2+8)+2, kind: fixup_7_pcrel
+; CHECK: brcs bar           ; encoding: [0bAAAAA000,0b111100AA]
+; CHECK-NEXT:               ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: 20 f0      brlo .+8
+; INST-NEXT: 10 f0      brlo .+4
+; INST-NEXT: 20 f0      brlo .+8
+; INST-NEXT: 00 f0      brlo .+0

llvm/test/MC/AVR/inst-breq.s

@@ -0,0 +1,28 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  breq .-18
+  breq .-12
+  brbs 1, .-18
+  brbs 1, bar
+
+bar:
+
+; CHECK: breq    (.Ltmp0-18)+2     ; encoding: [0bAAAAA001,0b111100AA]
+; CHECK-NEXT:                      ;   fixup A - offset: 0, value: (.Ltmp0-18)+2, kind: fixup_7_pcrel
+; CHECK: breq    (.Ltmp1-12)+2     ; encoding: [0bAAAAA001,0b111100AA]
+; CHECK-NEXT:                      ;   fixup A - offset: 0, value: (.Ltmp1-12)+2, kind: fixup_7_pcrel
+; CHECK: brbs    1, (.Ltmp2-18)+2  ; encoding: [0bAAAAA001,0b111100AA]
+; CHECK-NEXT:                      ;   fixup A - offset: 0, value: (.Ltmp2-18)+2, kind: fixup_7_pcrel
+; CHECK: brbs    1, bar            ; encoding: [0bAAAAA001,0b111100AA]
+; CHECK-NEXT:                      ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: b9 f3      breq .-18
+; INST-NEXT: d1 f3      breq .-12
+; INST-NEXT: b9 f3      breq .-18
+; INST-NEXT: 01 f0      breq .+0

llvm/test/MC/AVR/inst-brge.s

@@ -0,0 +1,24 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brge .+50
+  brge .+42
+  brge bar
+
+bar:
+
+; CHECK: brge (.Ltmp0+50)+2  ; encoding: [0bAAAAA100,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0+50)+2, kind: fixup_7_pcrel
+; CHECK: brge (.Ltmp1+42)+2  ; encoding: [0bAAAAA100,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1+42)+2, kind: fixup_7_pcrel
+; CHECK: brge bar            ; encoding: [0bAAAAA100,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: cc f4      brge .+50
+; INST-NEXT: ac f4      brge .+42
+; INST-NEXT: 04 f4      brge .+0

llvm/test/MC/AVR/inst-brhc.s

@@ -0,0 +1,24 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brhc .+12
+  brhc .+14
+  brhc bar
+
+bar:
+
+; CHECK: brhc (.Ltmp0+12)+2  ; encoding: [0bAAAAA101,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0+12)+2, kind: fixup_7_pcrel
+; CHECK: brhc (.Ltmp1+14)+2  ; encoding: [0bAAAAA101,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1+14)+2, kind: fixup_7_pcrel
+; CHECK: brhc bar            ; encoding: [0bAAAAA101,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: 35 f4      brhc .+12
+; INST-NEXT: 3d f4      brhc .+14
+; INST-NEXT: 05 f4      brhc .+0

llvm/test/MC/AVR/inst-brhs.s

@@ -0,0 +1,24 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brhs .-66
+  brhs .+14
+  brhs bar
+
+bar:
+
+; CHECK: brhs (.Ltmp0-66)+2  ; encoding: [0bAAAAA101,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0-66)+2, kind: fixup_7_pcrel
+; CHECK: brhs (.Ltmp1+14)+2  ; encoding: [0bAAAAA101,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1+14)+2, kind: fixup_7_pcrel
+; CHECK: brhs bar            ; encoding: [0bAAAAA101,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: fd f2      brhs .-66
+; INST-NEXT: 3d f0      brhs .+14
+; INST-NEXT: 05 f0      brhs .+0

llvm/test/MC/AVR/inst-brid.s

@@ -0,0 +1,24 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brid .+42
+  brid .+62
+  brid bar
+
+bar:
+
+; CHECK: brid (.Ltmp0+42)+2  ; encoding: [0bAAAAA111,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0+42)+2, kind: fixup_7_pcrel
+; CHECK: brid (.Ltmp1+62)+2  ; encoding: [0bAAAAA111,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1+62)+2, kind: fixup_7_pcrel
+; CHECK: brid bar            ; encoding: [0bAAAAA111,0b111101AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: af f4      brid .+42
+; INST-NEXT: ff f4      brid .+62
+; INST-NEXT: 07 f4      brid .+0

llvm/test/MC/AVR/inst-brie.s

@@ -0,0 +1,24 @@
+; RUN: llvm-mc -triple avr -show-encoding < %s | FileCheck %s
+;
+; RUN: llvm-mc -filetype=obj -triple avr < %s \
+; RUN:     | llvm-objdump -d - \
+; RUN:     | FileCheck --check-prefix=INST %s
+
+foo:
+  brie .+20
+  brie .+40
+  brie bar
+
+bar:
+
+; CHECK: brie (.Ltmp0+20)+2  ; encoding: [0bAAAAA111,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp0+20)+2, kind: fixup_7_pcrel
+; CHECK: brie (.Ltmp1+40)+2  ; encoding: [0bAAAAA111,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: (.Ltmp1+40)+2, kind: fixup_7_pcrel
+; CHECK: brie bar            ; encoding: [0bAAAAA111,0b111100AA]
+; CHECK-NEXT:                ;   fixup A - offset: 0, value: bar, kind: fixup_7_pcrel
+
+; INST-LABEL: <foo>:
+; INST-NEXT: 57 f0      brie .+20
+; INST-NEXT: a7 f0      brie .+40
+; INST-NEXT: 07 f0      brie .+0