[PATCH] D73739: Exception support for basic block sections

Tue Sep 29 11:14:59 PDT 2020

rahmanl added a comment.

In D73739#2299885 <https://reviews.llvm.org/D73739#2299885>, @MaskRay wrote:

> Let's take gcc_except_table_bb_sections.ll as the example:
>
> Call site sizes are incorrect
> -----------------------------
>
>   .uleb128 .Lcst_end0-.Lcst_begin0  # Call Site Size
>   
>   .uleb128 .Lcst_end0-.Lcst_begin1  # Call Site Size
>   
>   .uleb128 .Lcst_end0-.Lcst_begin2  # Call Site Size
>
> I think the Call Site Sizes are incorrect. A pair of .Lcst_end0 and .Lcst_begin0 should exactly delimiter the call sites table, but the current implements uses `.Lcst_end0` for all 3 basic block sections and places  `.Lcst_end0` after the last Call Site Table.

Yes. It's a bit confusing I agree. The label difference is used to locate the action table by the exception handling runtime. It also happens to be equal to the call site **table** size for no-basic-block-sections.
With basic block sections, these offsets do not correspond to the call site table. We have one `cst_end` label and multiple `cst_begin` labels. However, correctness is guaranteed because each call site range explicitly includes the LSDA headers.
One alternative is to change the name of these symbols similar to type table references (`cst_begin` to `atbaseref`  and   `cst_end` to `atbase`  where `at` represents Action Table) to make sure they're not incorrectly interpreted, but this will be applied to default too.

> Action table
> ------------
>
>> All these call-site tables will share their action and type tables.
>
> I think type tables can be shared but action tables cannot.
>
> In each call-site table, the action record offset of each call site record is relative to the action table the start of the action table. The action table follows the call-site table.
>
> "The action table follows the call-site table in the LSDA." If there are N call-site tables, there should be N corresponding action tables. See `libcxxabi/src/cxa_exceptions.cpp:scan_eh_tab` for how the table is parsed (`actionTableStart + (actionEntry - 1)`)

Even with basic-block sections, there is still only one call-site table, but multiple call-site ranges. This works because each call-site range explicitly specifies references to action table and type table. So `actionTableStart` across all call-site ranges.

> (Apologies that I only noticed this problem just now. I am learning exceptions while reviewing this patch...)

No worries. Thanks for taking the time to understand and review.

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https://reviews.llvm.org/D73739