[AMDGPU] Fix s_branch with -1 offset

[SamWot]

Summary

In case s_branch instruction target is itself backend should emit offset -1 but instead it emit 0.

label:
    s_branch label  // should emit [0xff,0xff,0x82,0xbf]

Tom, Matt: why are we adjusting fixup values in applyFixup() method instead of processFixup()? processFixup() is calling adjustFixupValue() but does nothing with its result.

[arsenm]

I changed this in r279895 to match what AArch64 does and report an error based on the context rather than just report_fatal_error

lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

141–143

I think this can be sunk down to the use

[SamWot]

Moved adjustFixupFalue() usage to processFixup() method

[arsenm]

LGTM

lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

 }
 
 void AMDGPUAsmBackend::processFixupValue(const MCAssembler &Asm,
                                          const MCAsmLayout &Layout,
                                          const MCFixup &Fixup, const MCFragment *DF,
                                          const MCValue &Target, uint64_t &Value,
                                          bool &IsResolved) {
   if (IsResolved)
-    (void)adjustFixupValue(Fixup, Value, &Asm.getContext());
-
+    Value = adjustFixupValue(Fixup, Value, &Asm.getContext());
 }
 
 void AMDGPUAsmBackend::applyFixup(const MCFixup &Fixup, char *Data,
                                   unsigned DataSize, uint64_t Value,
                                   bool IsPCRel) const {
-  unsigned NumBytes = getFixupKindNumBytes(Fixup.getKind());
   if (!Value)
     return; // Doesn't change encoding.
 
-  Value = adjustFixupValue(Fixup, Value, nullptr);
-
   MCFixupKindInfo Info = getFixupKindInfo(Fixup.getKind());
 
   // Shift the value into position.
   Value <<= Info.TargetOffset;
 
+  unsigned NumBytes = getFixupKindNumBytes(Fixup.getKind());
   uint32_t Offset = Fixup.getOffset();
   assert(Offset + NumBytes <= DataSize && "Invalid fixup offset!");
 
   // For each byte of the fragment that the fixup touches, mask in the bits from
   // the fixup value.
   for (unsigned i = 0; i != NumBytes; ++i)
     Data[Offset + i] |= static_cast<uint8_t>((Value >> (i * 8)) & 0xff);
 }

test/MC/AMDGPU/labels-branch.s

 // RUN: llvm-mc -arch=amdgcn -mcpu=fiji -show-encoding %s | FileCheck %s --check-prefix=VI
+// RUN: llvm-mc -arch=amdgcn -mcpu=fiji -filetype=obj %s | llvm-objdump -disassemble -mcpu=fiji - | FileCheck %s --check-prefix=BIN
 
 loop_start:
 s_branch loop_start
 // VI: s_branch loop_start ; encoding: [A,A,0x82,0xbf]
 // VI-NEXT: ;   fixup A - offset: 0, value: loop_start, kind: fixup_si_sopp_br
+// BIN: loop_start:
+// BIN-NEXT: BF82FFFF
 
 s_branch loop_end
 // VI: s_branch loop_end ; encoding: [A,A,0x82,0xbf]
 // VI-NEXT: ;   fixup A - offset: 0, value: loop_end, kind: fixup_si_sopp_br
+// BIN: BF820000
+// BIN: loop_end:
 loop_end:
 
 s_branch gds
 // VI: s_branch gds ; encoding: [A,A,0x82,0xbf]
 // VI-NEXT: ;   fixup A - offset: 0, value: gds, kind: fixup_si_sopp_br
+// BIN: BF820000
+// BIN: gds:
 gds:
   s_nop 0