Add support for variant calling conventions for lazily-bound functions

Currently, we save/restore a0-a7 and fa0-fa7 in the glibc and FreeBSD
run-time linker lazy resolvers to avoid clobbering any function
arguments. In order to define the vector calling convention, we need to
specify the behavior of the resolver in the dynamic linker. Vector
registers may have a large size and saving/restoring vector argument
registers may occupy a large portion of stack space during run-time
resolving. It is unreasonable to save/restore all these vector argument
registers in the resolver.

In this patch, we define STO_RISCV_VARIANT_CC, a special symbol
attribute to mark symbols as using a calling convention that passes
arguments in registers other than a0-a7 and fa0-fa7 (such as the vector
calling convention), which run-time linkers can use to eagerly bind such
functions. We also define DT_RISCV_VARIANT_CC, an additional dynamic tag
to indicate there are symbols with the special attribute in the dynamic
symbol table of the object used for PLT-based calls as a quick way for
run-time linkers to skip iterating over the table at load time when no
symbols need to be eagerly bound.

Co-authored-by: Jessica Clarke <[email protected]>

Author: Hsiangkai

riscv-elf.md

@@ -14,6 +14,7 @@
 	* [Named ABIs](#named-abis)
 	* [Default ABIs](#default-abis)
 	* [Code models](#code-models)
+	* [Dynamic Linking](#dynamic-linking)
 3. [C/C++ type details](#c-types)
 	* [C/C++ type sizes and alignments](#c-type-sizes)
 	* [C/C++ type representations](#c-type-representation)
@@ -28,7 +29,7 @@
 	* [Thread Local Storage](#thread-local-storage)
 	* [Program Header Table](#program-header-table)
 	* [Note Sections](#note-sections)
-	* [Dynamic Table](#dynamic-table)
+	* [Dynamic Section](#dynamic-section)
 	* [Hash Table](#hash-table)
 	* [Attributes](#Attributes)
 5. [DWARF](#dwarf)
@@ -371,6 +372,21 @@ a signed 32-bit offset, relative to the value of the `pc` register,
 can be produced.
 This code model is position independent.
 
+## <a name=dynamic-linking></a>Dynamic Linking
+
+Run-time linkers that use lazy binding must preserve all argument registers
+used in the standard calling convention for the ABI in use. Any functions that
+use additional argument registers must be annotated with
+`STO_RISCV_VARIANT_CC`, as defined in [Symbol Table](#symbol-table).
+
+> NOTE:
+> Vector registers have a variable size depending on the hardware
+> implementation and can be quite large. Saving/restoring all these vector
+> arguments in a run-time linker's lazy resolver would use a large amount of
+> stack space and hurt performance. This attribute allows vector registers to
+> not be part of the standard calling convention so run-time linkers are not
+> required to save/restore them and can instead eagerly bind such functions.
+
 # <a name=c-types></a>C/C++ type details
 
 ## <a name=c-type-sizes></a>C/C++ type sizes and alignments
@@ -519,7 +535,20 @@ There are no RISC-V specific definitions relating to ELF string tables.
 
 ## <a name=symbol-table></a>Symbol Table
 
-There are no RISC-V specific definitions relating to ELF symbol tables.
+* st_other: The lower 2 bits are used to specify a symbol's visibility. The
+  remaining 6 bits have no defined meaning in the ELF gABI. We use the highest
+  bit to mark functions that do not follow the standard calling convention for
+  the ABI in use.
+
+  The defined processor-specific `st_other` flags are listed in the following
+  table.
+
+  Name                 | Mask
+  :------------------- | :----
+  STO_RISCV_VARIANT_CC | 0x80
+
+  See [Dynamic Linking](#dynamic-linking) for the meaning of
+  `STO_RISCV_VARIANT_CC`.
 
 ## <a name=relocations></a>Relocations
 
@@ -1023,9 +1052,18 @@ PT_RISCV_ATTRIBUTES  | 0x70000003  | RISC-V ELF attribute section.
 
 There are no RISC-V specific definitions relating to ELF note sections.
 
-## <a name=dynamic-table></a>Dynamic Table
+## <a name=dynamic-section></a>Dynamic Section
+
+The defined processor-specific dynamic array tags are listed in the following
+table.
+
+Name                | Value      | d_un  | Executable        | Shared Object
+:------------------ | :--------- | :---- | :---------------- | :-----------------
+DT_RISCV_VARIANT_CC | 0x70000001 | d_val | Platform specific | Platform specific
 
-There are no RISC-V specific definitions relating to dynamic tables.
+An object with the dynamic tag `DT_RISCV_VARIANT_CC` has one or more
+`R_RISCV_JUMP_SLOT` relocations against symbols with the `STO_RISCV_VARIANT_CC`
+attribute.
 
 ## <a name=hash-table></a>Hash Table