Merge pull request #361 from riscv-non-isa/uleb128-reloc

Define relocations for ULEB128 value: R_RISCV_SET_ULEB128 and R_RISCV_SUB_ULEB128

Author: kito-cheng

riscv-elf.adoc

@@ -447,7 +447,11 @@ Description:: Additional information about the relocation
                                             <| `ifunc_resolver(B + A)`
 .2+| 59      .2+| PLT32         .2+| Static  | _word32_          .2+| 32-bit relative offset to a function or its PLT entry
                                             <| S + A - P
-.2+| 60-191  .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for future standard use
+.2+| 60      .2+| SET_ULEB128   .2+| Static  | _ULEB128_         .2+| Local label assignment <<uleb128-note,*note>>
+                                            <| S + A
+.2+| 61      .2+| SUB_ULEB128   .2+| Static  | _ULEB128_         .2+| Local label subtraction <<uleb128-note,*note>>
+                                            <| V - S + A
+.2+| 62-191  .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for future standard use
                                             <|
 .2+| 192-255 .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for nonstandard ABI extensions
                                             <|
@@ -460,6 +464,12 @@ This section and later ones contain fragments written in assembler. The precise
 assembler syntax, including that of the relocations, is described in the
 _RISC-V Assembly Programmer's Manual_ <<rv-asm>>.
 
+[[uleb128-note]]
+NOTE: The assembler must allocate enough space to accommodate ULEB128 data for
+`R_RISCV_SET_ULEB128` and `R_RISCV_SUB_ULEB128` relocations. This can be
+achieved either by padding with zeroes to fill up to the XLEN-bits boundary
+or by filling the precomputed result into the relocation location.
+
 ==== Calculation Symbols
 
 <<var-reloc-calc>> provides details on the variables used in relocation
@@ -501,6 +511,7 @@ of the `__global_pointer$` symbol into register `gp` (aka `x3`).
 | _word16_    | Specifies a 16-bit word
 | _word32_    | Specifies a 32-bit word
 | _word64_    | Specifies a 64-bit word
+| _ULEB128_   | Specifies a variable-length data encoded in ULEB128 format.
 | _wordclass_ | Specifies a _word32_ field for ILP32 or a _word64_ field for LP64
 | _B-Type_    | Specifies a field as the immediate field in a B-type instruction
 | _CB-Type_   | Specifies a field as the immediate field in a CB-type instruction
@@ -1221,6 +1232,14 @@ are commonly resolved at compile-time, such as intra-function jumps), code
 generators must in general ensure that relocations are always emitted when
 relaxation is enabled.
 
+Linkers should adjust relocations that refer to symbols whose addresses have
+been updated.
+
+ULEB128 value with relocation must be padding to the same length even if the
+data can be encoded with a shorter byte sequence after linker relaxation, The
+linker should report errors if the length of ULEB128 byte sequence is more
+extended than the current byte sequence.
+
 === Linker Relaxation Types
 
 The purpose of this section is to describe all types of linker relaxation,