Mapping symbol optimization: implicit one at section beginning

[Fangrui Song]

tl;dr Add gas option --[no-]optimize-mapping-symbols

Except the trivial cases (e.g. empty section), in all(?) gas ports
that support mapping symbols,

* A non-text section (data, debug, etc) almost always starts with an initial $d.
* A text section almost always starts with an initial $x (or the
equivalent for the port, e.g. $a/$t). aaelf32 and aaelf64 requires a
mapping symbol at offset 0.

This strategy can significantly inflate the symbol table, particularly
in 64-bit architectures (sizeof(Elf64_Sym) == 24) with larger
programs.
This issue becomes more pronounced when using -ffunction-sections
-fdata-sections, which generates numerous small sections.
An example is provided at the end of this message.

---

I have written a detailed analysis at
https://maskray.me/blog/2024-07-21-mapping-symbols-rethinking-for-efficiency
and proposed an alternative scheme to address the size concern:

* Text sections: Assume an implicit $x at offset 0. Add an ending $x
if the final data isn't instructions.
* Non-text sections: Assume an implicit $d at offset 0. Add an ending
$d only if the final data isn't data commands.

This approach eliminates most mapping symbols while ensuring correct
disassembly. Here is an illustrated assembler example:

.section .text.f0,"ax"
  ret
// emit $d
  .long 42
// emit $x. Without this, .text.f1 might be interpreted as data.

.section .text.f1,"ax"
  ret

However, omitting a mapping symbol at offset 0 for sections with
instructions is currently non-conformant for Arm. An ABI update has
been requested to address this
(https://github.com/ARM-software/abi-aa/issues/274). RISC-V ABI
doesn't have the requirement.

**While some interoperability concerns might arise, they're unlikely
to impact a majority of users.**

For instance, if a text section with trailing data is assembled using
the traditional behavior, the last mapping symbol will be $d. When
linked with another text section assembled using the new behavior
(lacking an initial $x), disassemblers might misinterpret the start of
the latter section as data.

Similarly, linker scripts that combine non-text and text sections
could lead to text sections appearing in a data state.

Ultimately, the proposed alternative scheme effectively addresses
symbol table bloat, but requires careful consideration for compliance
and interoperability.
I have thought about **an assembler option
--[no-]optimize-mapping-symbols**, which can be enabled by GCC as
-Wa,--optimize-mapping-symbols.
With this optimization enabled, most mapping symbols will be
eliminated for architectures that don't often place constant
pools/jump tables in text sections.
This optimization works well when there is a single code state (e.g.
AArch64) and can be extended when there are more states.

RISC-V $x<isa> poses some challenges:
https://maskray.me/blog/2024-07-21-mapping-symbols-rethinking-for-efficiency#risc-v-isa-extension


---

% cat a.c
void f0() {}
void f1() {}
void f2() {}
int d1 = 1;
int d2 = 2;
asm(
".pushsection .nonalloc,\n"
".balign 4\n"
".long 0\n"
".popsection\n"
);
% aarch64-linux-gnu-gcc -ffunction-sections -fdata-sections a.c
% llvm-objdump -d --show-all-symbols a.o  # GNU --show-all-symbols
doesn't display mapping symbols

a.o:    file format elf64-littleaarch64

Disassembly of section .text.f0:

0000000000000000 <$x>:
0000000000000000 <f0>:
       0: d503201f      nop
       4: d65f03c0      ret

Disassembly of section .text.f1:

0000000000000000 <$x>:
0000000000000000 <f1>:
       0: d503201f      nop
       4: d65f03c0      ret

Disassembly of section .text.f2:

0000000000000000 <$x>:
0000000000000000 <f2>:
       0: d503201f      nop
       4: d65f03c0      ret
% readelf -sX a.o

Symbol table '.symtab' contains 26 entries:
   Num:    Value          Size Type    Bind   Vis+Other Ndx(SecName)
Name [+ Version Info]
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS           a.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 (.text)   .text
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 (.data)   .data
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 (.bss)    .bss
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 (.text.f0) .text.f0
     6: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    4 (.text.f0) $x
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 (.text.f1) .text.f1
     8: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    5 (.text.f1) $x
     9: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 (.text.f2) .text.f2
    10: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    6 (.text.f2) $x
    11: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 (.data.d1) .data.d1
    12: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    7 (.data.d1) $d
    13: 0000000000000000     0 SECTION LOCAL  DEFAULT    8 (.data.d2) .data.d2
    14: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    8 (.data.d2) $d
    15: 0000000000000000     0 SECTION LOCAL  DEFAULT    9 (.nonalloc) .nonalloc
    16: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    9 (.nonalloc) $d
    17: 0000000000000000     0 SECTION LOCAL  DEFAULT   11
(.note.GNU-stack) .note.GNU-stack
    18: 0000000000000014     0 NOTYPE  LOCAL  DEFAULT   12 (.eh_frame) $d
    19: 0000000000000000     0 SECTION LOCAL  DEFAULT   12 (.eh_frame) .eh_frame
    20: 0000000000000000     0 SECTION LOCAL  DEFAULT   10 (.comment) .comment
    21: 0000000000000000     8 FUNC    GLOBAL DEFAULT    4 (.text.f0) f0
    22: 0000000000000000     8 FUNC    GLOBAL DEFAULT    5 (.text.f1) f1
    23: 0000000000000000     8 FUNC    GLOBAL DEFAULT    6 (.text.f2) f2
    24: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    7 (.data.d1) d1
    25: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    8 (.data.d2) d2