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Refer mapping symbol into R_RISCV_RELAX for rvc/norvc relaxations. #393

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Refer mapping symbol into R_RISCV_RELAX for rvc/norvc relaxations. #393

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34 changes: 33 additions & 1 deletion riscv-elf.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -201,6 +201,7 @@ below.

+
--
[[EF_RISCV_RVC]]
EF_RISCV_RVC (0x0001)::: This bit is set when the binary targets the C ABI,
which allows instructions to be aligned to 16-bit boundaries (the base RV32
and RV64 ISAs only allow 32-bit instruction alignment). When linking
Expand Down Expand Up @@ -430,7 +431,7 @@ Description:: Additional information about the relocation
<| S + A
.2+| 47-50 .2+| *Reserved* .2+| - | .2+| Reserved for future standard use
<|
.2+| 51 .2+| RELAX .2+| Static | .2+| Instruction can be relaxed, paired with a normal relocation at the same address
.2+| 51 .2+| RELAX .2+| Static | .2+| Instruction can be relaxed, paired with a normal relocation at the same address. This relocation might point to an optional dummy mapping symbol, indicating which extension is permitted for use during the linker relaxation process.
<|
.2+| 52 .2+| SUB6 .2+| Static | _word6_ .2+| Local label subtraction
<| V - S - A
Expand Down Expand Up @@ -1622,6 +1623,37 @@ instructions. It is recommended to initialize `jvt` CSR immediately after
csrw jvt, a0
----

=== Compressed and Non-compressed Relaxations in the Same Object

Linker used to enables and disables the compressed relaxations by checking the
[[EF_RISCV_RVC]] of each intput object. Since [[EF_RISCV_RVC]] is an
object-level tag, it cannot handle the case that if `.option arch, +c` and
`.option arch, -c` are in the same object. Therefore, encode the mapping symbols
into each R_RISCV_RELAX can resolve the problem.

Example::
+
--
Relaxation candidate:
[,asm]
----
.option arch, rv32i
auipc ra, 0 # R_RISCV_CALL_PLT (symbol), R_RISCV_RELAX ($xrv32i)
jalr ra, ra, 0

.option arch, +c
auipc ra, 0 # R_RISCV_CALL_PLT (symbol), R_RISCV_RELAX ($xrv32ic)
jalr ra, ra, 0
----
Relaxation result:
[,asm]
----
jal ra, 0 # R_RISCV_JAL (symbol)

c.jal ra, <offset-between-pc-and-symbol>
----
--

[bibliography]
== References

Expand Down
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