[llvm-dev] [tablegen] table readability / performance

[Reid Kleckner via llvm-dev]

I don't think there's any technical reason for the current structure. Your
change sounds like an improvement to me. I'm curious if you have any
compile time measurements from this change.

On Tue, Jan 14, 2020 at 4:59 AM Luke Drummond via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello
>
> I've been looking at the tables generated by
> `SequenceToOffsetTable::emit`, and notice that when the generated data
> are strings, the data is basically un-grep-able, and very tricky to
> read, as they are emitted as an array of comma-separated char-literal:
>
>     extern const char HexagonInstrNameData[] = {
>       /* 0 */ 'G', '_', 'F', 'L', 'O', 'G', '1', '0', 0,
>       /* 9 */ 'E', 'N', 'D', 'L', 'O', 'O', 'P', '0', 0,
>       /* 18 */ 'V', '6', '_', 'v', 'd', 'd', '0', 0,
>       /* 26 */ 'P', 'S', '_', 'v', 'd', 'd', '0', 0,
>       /* 34 */ 'V', '6', '_', 'l', 'd', '0', 0,
>       /* 41 */ 'V', '6', '_', 'z', 'l', 'd', '0', 0,
>       [...]
>     };
>
> As far as I can see, this makes it more difficult than necessary to read
> for at least the following cases:
>
>     Target AsmStrs
>     Target InstrNameData
>     Target RegStrings
>     Target RegClassStrings
>
>
> I hacked together a fix for the above cases locally, and found that for
> at least for clang and gcc, the compile-time for generated tables is
> significantly reduced when emitting string literals, and the user can
> grep the name tables without huge effort. The above table is now:
>
>     extern const char HexagonInstrNameData[] = {
>       /* 0 */ "G_FLOG10\0"
>       /* 9 */ "ENDLOOP0\0"
>       /* 18 */ "V6_vdd0\0"
>       /* 26 */ "PS_vdd0\0"
>       /* 34 */ "V6_ld0\0"
>       /* 41 */ "V6_zld0\0"
>       [...]
>     };
>
> My question then is: Is there a specific technical reason that we should
> avoid emitting concatenated string literals rather array of
> comma-separated char literals for "string-like" data?
>
> If not, I can probably post a patch, which I feel will make it much
> easier to understand the output from tablegen, and helps compilation
> speed of generated tables.
>
> Any thoughts appreciated.
>
> All the Best
>
> Luke
>
> --
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