[lld] [lld][AArch64] Fix handling of SHT_REL relocation addends. (PR #98291)
Peter Smith via llvm-commits
llvm-commits at lists.llvm.org
Thu Jul 11 14:19:31 PDT 2024
================
@@ -239,30 +239,85 @@ int64_t AArch64::getImplicitAddend(const uint8_t *buf, RelType type) const {
case R_AARCH64_IRELATIVE:
case R_AARCH64_TLS_TPREL64:
return read64(buf);
+
+ // The following relocation types all point at instructions, and
+ // relocate an immediate field in the instruction.
+ //
+ // The general rule, from AAELF64 ยง5.7.2 "Addends and PC-bias",
+ // says: "If the relocation relocates an instruction the immediate
+ // field of the instruction is extracted, scaled as required by
+ // the instruction field encoding, and sign-extended to 64 bits".
+
+ // The R_AARCH64_MOVW family operates on wide MOV/MOVK/MOVZ
+ // instructions, which have a 16-bit immediate field with its low
+ // bit in bit 5 of the instruction encoding. When the immediate
+ // field is used as an implicit addend for REL-type relocations,
+ // it is treated as added to the low bits of the output value, not
+ // shifted depending on the relocation type.
+ //
+ // This allows REL relocations to express the requirement 'please
+ // add 12345 to this symbol value and give me the four 16-bit
+ // chunks of the result', by putting the same addend 12345 in all
+ // four instructions. Carries between the 16-bit chunks are
+ // handled correctly, because the whole 64-bit addition is done
+ // once per relocation.
case R_AARCH64_MOVW_UABS_G0:
case R_AARCH64_MOVW_UABS_G0_NC:
- return getBits(SignExtend64<16>(read16(buf)), 0, 15);
+ return SignExtend64<16>(getBits(read32(buf), 5, 20));
case R_AARCH64_MOVW_UABS_G1:
case R_AARCH64_MOVW_UABS_G1_NC:
- return getBits(SignExtend64<32>(read32(buf)), 16, 31);
+ return SignExtend64<16>(getBits(read32(buf), 5, 20));
case R_AARCH64_MOVW_UABS_G2:
case R_AARCH64_MOVW_UABS_G2_NC:
- return getBits(read64(buf), 32, 47);
+ return SignExtend64<16>(getBits(read32(buf), 5, 20));
case R_AARCH64_MOVW_UABS_G3:
- return getBits(read64(buf), 48, 63);
+ return SignExtend64<16>(getBits(read32(buf), 5, 20));
+
+ // R_AARCH64_TSTBR14 points at a TBZ or TBNZ instruction, which
+ // has a 14-bit offset measured in instructions, i.e. shifted left
+ // by 2.
case R_AARCH64_TSTBR14:
- return getBits(SignExtend64<32>(read32(buf)), 2, 15);
+ return SignExtend64<16>(getBits(read32(buf), 5, 18) << 2);
+
+ // R_AARCH64_CONDBR19 operates on the ordinary B.cond instruction,
+ // which has a 19-bit offset measured in instructions.
case R_AARCH64_CONDBR19:
+ return SignExtend64<21>(getBits(read32(buf), 5, 23) << 2);
+
+ // R_AARCH64_LD_PREL_LO19 operates on the LDR (literal)
+ // instruction, which also has a 19-bit offset, measured in 4-byte
+ // chunks. So the calculation is the same as for
+ // R_AARCH64_CONDBR19.
case R_AARCH64_LD_PREL_LO19:
- return getBits(SignExtend64<32>(read32(buf)), 2, 20);
+ return SignExtend64<21>(getBits(read32(buf), 5, 23) << 2);
----------------
smithp35 wrote:
We could follow the same pattern as ADRP and ADR below and have both comments followed by:
```
case R_AARCH64_CONDBR19:
case R_AARCH64_LD_PREL_LO19:
return SignExtend64<21>(getBits(read32(buf), 5, 23) << 2);
```
https://github.com/llvm/llvm-project/pull/98291
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