riscv-elf.md: add new definitions for the compact code model #154
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@@ -510,65 +510,81 @@ Global Offset Table or DWARF meta data. | |
| The following table provides details of the RISC-V ELF relocations (instruction | ||
| specific relocations show the instruction type in the Details column): | ||
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| Enum | ELF Reloc Type | Description | Field | Calculation | Details | ||
| :--- | :------------- | :---------- | :---- | :---------- | :------ | ||
| 0 | R_RISCV_NONE | None | | ||
| 1 | R_RISCV_32 | Runtime relocation | _word32_ | S + A | ||
| 2 | R_RISCV_64 | Runtime relocation | _word64_ | S + A | ||
| 3 | R_RISCV_RELATIVE | Runtime relocation | _wordclass_ | B + A | ||
| 4 | R_RISCV_COPY | Runtime relocation | | | Must be in executable. not allowed in shared library | ||
| 5 | R_RISCV_JUMP_SLOT | Runtime relocation | _wordclass_ | S | Handled by PLT unless `LD_BIND_NOW` | ||
| 6 | R_RISCV_TLS_DTPMOD32 | TLS relocation | _word32_ | S->TLSINDEX | ||
| 7 | R_RISCV_TLS_DTPMOD64 | TLS relocation | _word64_ | S->TLSINDEX | ||
| 8 | R_RISCV_TLS_DTPREL32 | TLS relocation | _word32_ | S + A + TLS - TLS_TP_OFFSET | ||
| 9 | R_RISCV_TLS_DTPREL64 | TLS relocation | _word64_ | S + A + TLS - TLS_TP_OFFSET | ||
| 10 | R_RISCV_TLS_TPREL32 | TLS relocation | _word32_ | S + A + TLS + S_TLS_OFFSET - TLS_DTV_OFFSET | ||
| 11 | R_RISCV_TLS_TPREL64 | TLS relocation | _word64_ | S + A + TLS + S_TLS_OFFSET - TLS_DTV_OFFSET | ||
| 16 | R_RISCV_BRANCH | PC-relative branch | _B-Type_ | S + A - P | ||
| 17 | R_RISCV_JAL | PC-relative jump | _J-Type_ | S + A - P | ||
| 18 | R_RISCV_CALL | PC-relative call | _J-Type_ | S + A - P | Macros `call`, `tail` | ||
| 19 | R_RISCV_CALL_PLT | PC-relative call (PLT) | _J-Type_ | S + A - P | Macros `call`, `tail` (PIC) | ||
| 20 | R_RISCV_GOT_HI20 | PC-relative GOT reference | _U-Type_ | G + A | `%got_pcrel_hi(symbol)` | ||
| 21 | R_RISCV_TLS_GOT_HI20 | PC-relative TLS IE GOT offset | _U-Type_ | | Macro `la.tls.ie` | ||
| 22 | R_RISCV_TLS_GD_HI20 | PC-relative TLS GD reference | _U-Type_ | | Macro `la.tls.gd` | ||
| 23 | R_RISCV_PCREL_HI20 | PC-relative reference | _U-Type_ | S + A - P | `%pcrel_hi(symbol)` | ||
| 24 | R_RISCV_PCREL_LO12_I | PC-relative reference | _I-type_ | S + A - P | `%pcrel_lo(address of %pcrel_hi)` | ||
| 25 | R_RISCV_PCREL_LO12_S | PC-relative reference | _S-Type_ | S + A - P | `%pcrel_lo(address of %pcrel_hi)` | ||
| 26 | R_RISCV_HI20 | Absolute address | _U-Type_ | S + A | `%hi(symbol)` | ||
| 27 | R_RISCV_LO12_I | Absolute address | _I-Type_ | S + A | `%lo(symbol)` | ||
| 28 | R_RISCV_LO12_S | Absolute address | _S-Type_ | S + A | `%lo(symbol)` | ||
| 29 | R_RISCV_TPREL_HI20 | TLS LE thread offset | _U-Type_ | | `%tprel_hi(symbol)` | ||
| 30 | R_RISCV_TPREL_LO12_I | TLS LE thread offset | _I-Type_ | | `%tprel_lo(symbol)` | ||
| 31 | R_RISCV_TPREL_LO12_S | TLS LE thread offset | _S-Type_ | | `%tprel_lo(symbol)` | ||
| 32 | R_RISCV_TPREL_ADD | TLS LE thread usage | | | `%tprel_add(symbol)` | ||
| 33 | R_RISCV_ADD8 | 8-bit label addition | _word8_ | V + S + A | ||
| 34 | R_RISCV_ADD16 | 16-bit label addition | _word16_ | V + S + A | ||
| 35 | R_RISCV_ADD32 | 32-bit label addition | _word32_ | V + S + A | ||
| 36 | R_RISCV_ADD64 | 64-bit label addition | _word64_ | V + S + A | ||
| 37 | R_RISCV_SUB8 | 8-bit label subtraction | _word8_ | V - S - A | ||
| 38 | R_RISCV_SUB16 | 16-bit label subtraction | _word16_ | V - S - A | ||
| 39 | R_RISCV_SUB32 | 32-bit label subtraction | _word32_ | V - S - A | ||
| 40 | R_RISCV_SUB64 | 64-bit label subtraction | _word64_ | V - S - A | ||
| 41 | R_RISCV_GNU_VTINHERIT | GNU C++ vtable hierarchy | | ||
| 42 | R_RISCV_GNU_VTENTRY | GNU C++ vtable member usage | | ||
| 43 | R_RISCV_ALIGN | Alignment statement | | ||
| 44 | R_RISCV_RVC_BRANCH | PC-relative branch offset | _CB-Type_ | S + A - P | ||
| 45 | R_RISCV_RVC_JUMP | PC-relative jump offset | _CJ-Type_ | S + A - P | ||
| 46 | R_RISCV_RVC_LUI | Absolute address | _CI-Type_ | S + A | ||
| 47 | R_RISCV_GPREL_I | GP-relative reference | _I-Type_ | S + A - GP | ||
| 48 | R_RISCV_GPREL_S | GP-relative reference | _S-Type_ | S + A - GP | ||
| 49 | R_RISCV_TPREL_I | TP-relative TLS LE load | _I-Type_ | ||
| 50 | R_RISCV_TPREL_S | TP-relative TLS LE store | _S-Type_ | ||
| 51 | R_RISCV_RELAX | Instruction pair can be relaxed | | ||
| 52 | R_RISCV_SUB6 | Local label subtraction | _word6_ | V - S - A | ||
| 53 | R_RISCV_SET6 | Local label assignment | _word6_ | S + A | ||
| 54 | R_RISCV_SET8 | Local label assignment | _word8_ | S + A | ||
| 55 | R_RISCV_SET16 | Local label assignment | _word16_ | S + A | ||
| 56 | R_RISCV_SET32 | Local label assignment | _word32_ | S + A | ||
| 57 | R_RISCV_32_PCREL | PC-relative reference | _word32_ | S + A - P | ||
| 58 | R_RISCV_IRELATIVE | Runtime relocation | _wordclass_ | `ifunc_resolver(B + A)` | ||
| 59 | R_RISCV_64_PCREL | PC-relative reference | _word64_ | S + A - P | ||
| 60 | R_RISCV_GPREL_HI20 | GP-relative reference | _U-type_ | S + A - GP | `%gprel_hi(symbol)` | ||
| 61 | R_RISCV_GPREL_LO12_I | GP-relative reference | _I-type_ | S + A - GP | `%gprel_lo(symbol)` | ||
| 62 | R_RISCV_GPREL_LO12_S | GP-relative reference | _S-type_ | S + A - GP | `%gprel_lo(symbol)` | ||
| 63 | R_RISCV_GPREL_ADD | GP-relative usage | | | `%gprel_add(symbol)` | ||
| 64 | R_RISCV_GOT_GPREL_HI20 | GP-relative GOT reference | _U-type_ | G + A - GP | `%got_gprel_hi(symbol)` | ||
| 65 | R_RISCV_GOT_GPREL_LO12_I | GP-relative GOT reference | _I-type_ | G + A - GP | `%got_gprel_lo(symbol)` | ||
| 66 | R_RISCV_GOT_GPREL_ADD | GP-relative GOT usage | | | `%got_gprel_add(symbol)` | ||
| 67 | R_RISCV_GOT_GPREL_LOAD | GP-relative GOT usage | | | `%got_gprel(symbol)` | ||
| 68 | R_RISCV_GOT_GPREL_STORE | GP-relative GOT usage | | | `%got_gprel(symbol)` | ||
| 69 | R_RISCV_TLS_GOT_GPREL_HI20 | GP-relative TLS GOT reference | | | Macro `la.tls.ie.gprel` | ||
| 70 | R_RISCV_TLS_GOT_GPREL_LO20_I | GP-relative TLS GOT reference | | | Macro `la.tls.ie.gprel` | ||
| 71 | R_RISCV_TLS_GOT_GPREL_ADD | GP-relative TLS GOT usage | | | `%tls_ie_gprel(<symbol>)` | ||
| 72 | R_RISCV_TLS_GD_GPREL_HI20 | GP-relative TLS GD reference | | | Macro `la.tls.gd.gprel` | ||
| 73 | R_RISCV_TLS_GD_GPREL_LO20_I | GP-relative TLS GD reference | | | Macro `la.tls.gd.gprel` | ||
| 74 | R_RISCV_TLS_GD_GPREL_ADD | GP-relative TLS GD usage | | | `%tls_gd_gprel(<symbol>)` | ||
| 75-191 | *Reserved* | Reserved for future standard use | ||
| 192-255 | *Reserved* | Reserved for nonstandard ABI extensions | ||
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| Nonstandard extensions are free to use relocation numbers 192-255 for any | ||
| purpose. These relocations may conflict with other nonstandard extensions. | ||
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@@ -683,8 +699,10 @@ GOT offsets lazily on the first call to any function, except when | |
| `LD_BIND_NOW` is set in which case the GOT entries are populated by the | ||
| dynamic linker before the executable is started. Lazy resolution of GOT | ||
| entries is intended to speed up program loading by deferring symbol | ||
| resolution to the first time the function is called. | ||
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| For the small and medium code models, the first entry in the PLT occupies | ||
| two 16 byte entries: | ||
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| ``` | ||
| 1: auipc t2, %pcrel_hi(.got.plt) | ||
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@@ -697,18 +715,51 @@ in the PLT occupies two 16 byte entries: | |
| jr t3 | ||
| ``` | ||
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| For the compact code model, the first entry in the PLT has a stub that | ||
| calculates the absolute address of a function pointer in the GOT. | ||
| It occupies three 16 byte entries: | ||
|
Collaborator
There was a problem hiding this comment. Not enough space for RV128 with the current scheme, though arguable whether it's a meaningful combination. |
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| ``` | ||
| 1: auipc t0, %hi_pcrel(2f) # address of 2f | ||
| addi t0, %lo_pcrel(1b) | ||
| ld t2, (t0) # difference between .got.plt - 2f | ||
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| add t0, t0, t2 # address of .got.plt | ||
| add t0, t0, t3 # address of the function pointer | ||
| ld t3, (t0) # load the function pointer | ||
| jr t3 | ||
| nop | ||
| nop | ||
| nop | ||
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Collaborator
There was a problem hiding this comment. The number of
Contributor
Author
There was a problem hiding this comment. The compact code model does not apply to RV32, unless you mean RV128. |
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| 2: .quad .got.plt - ., 0 | ||
| ``` | ||
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| For the small and medium code models, subsequent function entries in the PLT | ||
| take up 16 bytes and load a function pointer from the GOT. | ||
| On the first call to a function, the entry redirects to the first PLT entr | ||
| which calls `_dl_runtime_resolve` and fills in the GOT entry | ||
| for subsequent calls to the function: | ||
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| ``` | ||
| 1: auipc t3, %pcrel_hi([email protected]) # address of the function pointer | ||
| l[w|d] t3, %pcrel_lo(1b)(t3) # dereference the function pointer | ||
| jalr t1, t3 | ||
| nop | ||
| ``` | ||
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| For the compact code model, subsequent function entries in the PLT | ||
| take up 16 bytes and load a function pointer from the GOT. | ||
| On the first call to a function, the entry redirects to the first PLT entr | ||
| which calls `_dl_runtime_resolve` and fills in the GOT entry | ||
| for subsequent calls to the function: | ||
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| ``` | ||
| 1: lui t3, %hi([email protected] - .got.plt) # offset to the function pointer | ||
| addi t3, %lo([email protected] - .got.plt) | ||
| jal t1, [email protected] | ||
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Collaborator
There was a problem hiding this comment. Wouldn't this all be a lot simpler if you just required that
Contributor
Author
There was a problem hiding this comment. Shared objects do not have a valid |
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| nop | ||
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| ``` | ||
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| ### Procedure Calls | ||
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@@ -904,6 +955,22 @@ label: | |
| {ld,lw} a5, 0(a5) # R_RISCV_PCREL_LO12_I (label) | ||
| ``` | ||
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| For the compact code model, | ||
| the pseudoinstruction `la.tls.ie.gprel` is used instead: | ||
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| ``` | ||
| la.tls.ie.gprel a5, symbol, gp | ||
| ``` | ||
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| Assuming that the `gp` register holds the value of `__global_pointer$`, | ||
| it expands to the following assembly instructions and relocations: | ||
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| ``` | ||
| lui a5, %tls_ie_gprel_hi(symbol) # R_RISCV_TLS_GOT_GPREL_HI20 (symbol) | ||
| add a5, gp, a5, %tls_ie_gprel(symbol) # R_RISCV_TLS_GOT_GPREL_ADD (symbol) | ||
| addi a5, a5, %tls_ie_gprel_lo(symbol) # R_RISCV_TLS_GOT_GPREL_LO12_I (symbol) | ||
| ``` | ||
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| ### Global Dynamic | ||
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@@ -943,6 +1010,22 @@ label: | |
| addi a0,a0,0 # R_RISCV_PCREL_LO12_I (label) | ||
| ``` | ||
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| For the compact code model, | ||
| the pseudoinstruction `la.tls.gd.gprel` is used instead: | ||
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| ``` | ||
| la.tls.gd.gprel a5, symbol, t0 | ||
| ``` | ||
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| Assuming that the `t0` register holds the value of `__global_pointer$`, | ||
| it expands to the following assembly instructions and relocations: | ||
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| ``` | ||
| lui a5, %tls_gd_gprel_hi(symbol) # R_RISCV_TLS_GD_GPREL_HI20 (symbol) | ||
| add a5, gp, a5, %tls_gd_gprel(symbol) # R_RISCV_TLS_GD_GPREL_ADD (symbol) | ||
| addi a5, a5, %tls_gd_gprel_lo(symbol) # R_RISCV_TLS_GD_GPREL_LO12_I (symbol) | ||
| ``` | ||
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| In the Global Dynamic model, the runtime library provides the `__tls_get_addr` function: | ||
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| ``` | ||
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Unrelated?
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It's used in the first PLT entry for the compact code model.