[CFIFixup] Fixup CFI for split functions with synchronous uwtables #125299
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dhoekwater
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Commit 6e54fcc disables CFI fixup for functions with synchronous tables, breaking CFI for split functions. Instead, we can disable *block-level* CFI fixup for functions with synchronous tables. Unwind tables can be: - N/A (not present) - Asynchronous - Synchronous Functions without unwind tables don't need CFI fixup (since they don't care about CFI). Functions with asynchronous unwind tables must be accurate for each basic block, so full CFI fixup is necessary. Functions with synchronous unwind tables only need to be accurate for each function (specifically, the portion of a function in a given section). Disabling CFI fixup entirely for functions with synchronous uwtables may break CFI for a function split between two sections. The portion in the first section may have valid CFI, while the portion in the second section is missing a call frame. Ex: ``` (.text.hot) Foo (BB1): <Call frame information> ... BB2: ... (.text.split) BB3: ... BB4: <epilogue> ``` Even if `Foo` has a synchronous unwind table, we still need to insert call frame information into `BB3` so that unwinding the call stack from `BB3` or `BB4` works properly.
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Commit 6e54fcc disables CFI fixup for
functions with synchronous tables, breaking CFI for split functions.
Instead, we can disable block-level CFI fixup for functions with
synchronous tables.
Unwind tables can be:
Functions without unwind tables don't need CFI fixup (since they don't
care about CFI).
Functions with asynchronous unwind tables must be accurate for each
basic block, so full CFI fixup is necessary.
Functions with synchronous unwind tables only need to be accurate for
each function (specifically, the portion of a function in a given
section). Disabling CFI fixup entirely for functions with synchronous
uwtables may break CFI for a function split between two sections. The
portion in the first section may have valid CFI, while the portion in
the second section is missing a call frame.
Ex:
Even if
Foohas a synchronous unwind table, we still need to insertcall frame information into
BB3so that unwinding the call stack fromBB3orBB4works properly.