[llvm-dev] RFC: Adding a string table to the bitcode format

Peter Collingbourne via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 3 19:08:08 PDT 2017


As part of PR27551 I want to add a string table to the bitcode format to
allow global value and comdat names to be shared with the proposed symbol
table (and, as side effects, allow comdat names to be shared with value
names, make bitcode files more compressible and make bitcode easier to
parse). The format of the string table would be a top-level block
containing a blob containing null-terminated strings [0] similar to the
string table format used in most object files.

records would change so that their first operand would specify their names
with a byte offset into the string table. (To allow for backwards
compatibility, I would increment the bitcode version.) Here is what it
would look like as bcanalyzer output:

  <VERSION op0=2>
  <COMDAT op0=0 ...> ; name = foo
  <FUNCTION op0=0 ...> ; name = foo
  <GLOBALVAR op0=4 ...> ; name = bar
  <ALIAS op0=8 ...> ; name = baz
 ; function bodies, etc.
  <STRTAB_BLOB blob="foo\0bar\0baz\0">

Each STRTAB_BLOCK would apply to all preceding MODULE_BLOCKs. This means
that bitcode files can continue to be concatenated with "llvm-cat -b".
(Normally bitcode files would contain a single string table, which in
multi-module bitcode files would be shared between modules.)

This *almost* allows us to remove the global (top-level) VST entirely, if
not for the function offset in the FNENTRY record. However, this offset is
not actually required because we can scan the module's FUNCTION_BLOCK_IDs
as we were doing before http://reviews.llvm.org/D12536 (this may have a
performance impact, so I'll measure it first).

Assuming that performance looks good, does this seem reasonable to folks?


[0] This means that no GlobalValue or comdat name can contain a null, but
this isn't substantially more restrictive than what we already have. The
restriction already exists in the form of an assert for value names
containing nulls (but not comdats) and we reject value and comdat names
containing nulls in the textual IR parser. The COFF/ELF/MachO object
formats do not support nulls in symbol or comdat names. The wasm format
could in principle support nulls in symbol names, but as mentioned we
already reject nulls early for IR symbols and wasm does not support comdats
(yet). The missing restriction on comdat names seems to be just a bug, so
we could assert on comdat names containing nulls as well.
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