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Add new relocation type for global array accesses with non-constant indices. #489

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Add new relocation type for global array accesses with non-constant indices. #489

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Add new relocation type for global array accesses with non-constant i…
LiqinWeng Mar 19, 2026
6aa375f
fix the comments
LiqinWeng Mar 20, 2026
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63 changes: 62 additions & 1 deletion riscv-elf.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -590,7 +590,15 @@ Description:: Additional information about the relocation
<|
.2+| 67-76 .2+| *Reserved* .2+| - | .2+| Reserved for Y base ISA
<|
.2+| 77-190 .2+| *Reserved* .2+| - | .2+| Reserved for future standard use
.2+| 77 .2+| REGREL_LO12_I .2+| Static | _I-Type_ .2+| Low 12 bits of 32-bit Global Array Offset, `%regrel_lo(symbol)`
<|
.2+| 78 .2+| REGREL_LO12_S .2+| Static | _S-Type_ .2+| Low 12 bits of 32-bit Global Array Offset, `%regrel_lo(symbol)`
<|
.2+| 79 .2+| REGREL_ADD .2+| Static | .2+| Global Array usage, `%regrel_add(symbol)`
<|
.2+| 80 .2+| REGREL_SHXADD .2+| Static | .2+| Global Array usage, `%regrel_shxadd(symbol)`
<|
.2+| 81-190 .2+| *Reserved* .2+| - | .2+| Reserved for future standard use
<|
.2+| 191 .2+| VENDOR .2+| Static | .2+| Paired with a vendor-specific relocation and must be placed immediately before it, indicates which vendor owns the relocation.
<|
Expand Down Expand Up @@ -1880,6 +1888,59 @@ Relaxation result:
----
--

==== Global Array Offset Relaxation

Target Relocation:: R_RISCV_HI20, R_RISCV_REGREL_LO12_I, R_RISCV_REGREL_LO12_S,
R_RISCV_REGREL_ADD, R_RISCV_REGREL_SHXADD
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@lenary lenary Mar 19, 2026

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Please split off the R_RISCV_REGREL_SHXADD relaxation into a separate description, or add a shxadd example below (either way, an example is needed, as that relaxation is more complex).

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@LiqinWeng LiqinWeng Mar 20, 2026

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done~


Description:: This relaxation type can relax a sequence of the load
address of a symbol or load/store with a symbol reference into shorter
instruction sequence if possible.

Condition:: Global-pointer relaxation requires that Tag_RISCV_x3_reg_usage
must be 0 or 1, and offset between global-pointer and symbol is within +-2KiB.
`R_RISCV_REGREL_SHXADD` the Zba feature is required.

Relaxation::
- Instruction associated with `R_RISCV_HI20` can be removed.

- Instruction associated with `R_RISCV_REGREL_ADD` or `R_RISCV_REGREL_SHXADD`
can be replaced with a global-pointer-relative access instruction. Update the
`R_RISCV_REGREL_LO12_I` or `R_RISCV_REGREL_LO12_S` Symbol's Offset

Example::
+
--
Relaxation candidate 1:
[,asm]
----
lui tX, 0 # R_RISCV_HI20 (symbol), R_RISCV_RELAX
add tY, tY, tX # R_RISCV_REGREL_ADD (symbol), R_RISCV_RELAX
lw t1, 0(tY) # R_RISCV_REGREL_LO12_I (symbol), R_RISCV_RELAX
----
Relaxation result:
[,asm]
----
add tY, tY, gp
lw t1, <gp-offset-for-symbol>(tY)
----
+
--
Relaxation candidate 2: same as sh2add/sh3add
[,asm]
----
lui tX, 0 # R_RISCV_HI20 (symbol), R_RISCV_RELAX
sh1add tY, tY, tX # R_RISCV_REGREL_SHXADD (symbol), R_RISCV_RELAX
lw t1, 0(tY) # R_RISCV_REGREL_LO12_I (symbol), R_RISCV_RELAX
----
Relaxation result:
[,asm]
----
sh1add tY, tY, gp
lw t1, <gp-offset-for-symbol>(tY)
----
--

==== Compressed LUI Relaxation

Target Relocation:: R_RISCV_HI20, R_RISCV_LO12_I, R_RISCV_LO12_S
Expand Down
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