[Bitcode] Mention that a multi-module bitcode is valid, each module has exactly one MODULE_BLOCK block

[mingmingl]

Update bitcode format to mention multi-module bitcode.

[tejohnson]

lgtm on the documentation change but that should be submitted separately as it is unrelated to the code fix.
The other change lgtm but if it is feasible to add a test that would be ideal.

llvm/docs/BitCodeFormat.rst

560

Can you fix the line length? The file seems to stick with 80 col.

llvm/lib/Bitstream/Reader/BitstreamReader.cpp

100 ↗

(On Diff #399709)

Is it feasible to add a test? What was the behavior without the fix?

[mingmingl]

Keep the documentation change and put bitcode reader change to https://reviews.llvm.org/D117630 (which is still a draft).

[mingmingl]

In D117067#3245194, @tejohnson wrote:

lgtm on the documentation change but that should be submitted separately as it is unrelated to the code fix.
The other change lgtm but if it is feasible to add a test that would be ideal.

thanks! I revise this patch to be doc-only, and reformat, and going to pursue bitcode reader change in https://reviews.llvm.org/D117630

The patch is still a draft and I'll see how to add a test (probably an invalid bitcode is needed to trigger the condition);

What's the behavior without the fix

I think upon hitting the branch, trying to call get will run into assertions (with the asserted condition being false); yet it happens very rarely (if at all) since most of time, bitcode file should be valid.

I'll see if there is a lightweight way to construct the bitcode to have a test (maybe by hacking bitcode writer to generate an invalid record).

[tejohnson]

lgtm

llvm/docs/BitCodeFormat.rst

 * 23 --- `STRTAB_BLOCK`_ --- The bitcode file's string table.
 
 .. _MODULE_BLOCK:
 
 MODULE_BLOCK Contents
 ---------------------
 
 The ``MODULE_BLOCK`` block (id 8) is the top-level block for LLVM bitcode files,
-and each bitcode file must contain exactly one. In addition to records
-(described below) containing information about the module, a ``MODULE_BLOCK``
-block may contain the following sub-blocks:
+and each module in a bitcode file must contain exactly one. A bitcode file with
+multi-module bitcode is valid. In addition to records (described below)
+containing information about the module, a ``MODULE_BLOCK`` block may contain
+the following sub-blocks:
 
 * `BLOCKINFO`_
 * `PARAMATTR_BLOCK`_
 * `PARAMATTR_GROUP_BLOCK`_
 * `TYPE_BLOCK`_
 * `VALUE_SYMTAB_BLOCK`_
 * `CONSTANTS_BLOCK`_
 * `FUNCTION_BLOCK`_