Sometimes we need to know the size of a symbol besides its address, so
maybe we can start using the existing `BOLTLinker::lookupSymbolInfo()`
(that returns symbol address and size) and remove
`BOLTLinker::lookupSymbol()` (that only returns symbol address). And for
both we need to check return value as it is wrapped in `std::optional<>`,
which makes the difference even smaller.
Traces are triplets of branch source, target, and fall-through end (next
branch).
Traces simplify differentiation of fall-throughs into local- and
external-origin, which improves performance over profile with
undifferentiated fall-throughs by eliminating profile discontinuity in
call to continuation fall-throughs. This makes it possible to avoid
converting return profile into call to continuation profile which may
introduce statistical biases.
The existing format makes provisions for local- (F) and external- (f)
origin fall-throughs, but the profile producer needs to know function
boundaries. BOLT has that information readily available, so providing
the origin branch of a fall-through is a functional replacement of the
fall-through kind (f or F). This also has an effect of combining
branches and fall-throughs into a single record.
As traces subsume other pre-aggregated profile kinds, BOLT may drop
support for them soon. Users of pre-aggregated profile format are
advised to migrate to the trace format.
Test Plan: Updated callcont-fallthru.s
When printing disassembly of a function with constant islands, include
the island info in the dump.
At the moment, only print islands in pre-CFG state. Include islands that
are interleaved with instructions.
When a function has an indirect branch with unknown control flow, we
preserve nops in order to keep all instruction offsets (from the start
of the function) the same in case the indirect branch is used by a
PC-relative jump table. However, when we know the control flow of the
function, we should be able to safely remove nops.
The code for jump table detection on AArch64 asserts liberally whenever
the input instruction sequence does not match the expected pattern. As a
result, BOLT fails to process binaries with such sequences instead of
ignoring functions with unknown control flow.
Remove asserts in analyzeIndirectBranchFragment() and mark indirect
jumps as instructions with unknown control flow instead.
Remove options to generate autofdo data (unused) and `use-event-pc`
(not beneficial).
Cuts down perf2bolt time for 11GB perf.data by 40s (11:10->10:30).
Bolt makes use of add_llvm_library and as such ends up exporting its
libraries from LLVMExports.cmake, which is not correct.
Bolt doesn't have its own exports file, and I assume that there is no
desire to have one either -- Bolt libraries are not intended to be
consumed as a cmake module, right?
As such, this PR adds a NO_EXPORT option to simplify exclude these
libraries from the exports file.
- **Reapply "[BOLT] Add --pad-funcs-before=func:n (#117924)"**
- **[BOLT] Fix --pad-funcs{,-before} state misinteraction**
When --pad-funcs-before was introduced, it introduced a bug whereby the
first one to get parsed could influence the other.
Ensure that each has its own state and test that they don't interact in
this manner by testing how the `_subsequent` symbol moves when both
arguments are supplied with different padding values.
Fixed by having a function (and static state) for each of before/after.
14dcf8214f9c66172d17c1cfaec6aec0030748e0 introduced a subtle bug with
the static `FunctionPadding` map.
If either `opts::FunctionPadSpec` or `opts::FunctionPadBeforeSpec` are set,
the map is going to be populated with the respective spec in the first
invocation of `BinaryEmitter::emitFunction`. The subsequent invocations
will pick up the padding from the map irrespective of whether
`opts::FunctionPadSpec` or `opts::FunctionPadBeforeSpec` is passed as a
parameter.
This breaks an internal test, hence reverting the patch.
This patch makes sure that `BinaryContext::printInstruction` prints the
preferred disassembly. Preferred disassembly only gets printed when
there are no annotations on the MCInst. Therefore, this patch
temporarily removes the annotations before printing it.
A few examples of before and after on AArch64 instructions are as
follows:
```
BEFORE AFTER
(preferred disassembly)
ret x30 ret
orr x30, xzr, x0 mov x30, x0
hint #29 autiasp
hint #12 autia1716
```
Clearly, the preferred disassembly is easier for developers to read, and
is the disassembly that tools should be printing.
This patch is motivated as part of future work on the
llvm-bolt-binary-analysis tool, making sure that the reports it prints
do use preferred disassembly.
This patch was cherry-picked from
https://github.com/kbeyls/llvm-project/tree/bolt-gadget-scanner-prototype.
In this current patch, this only affects existing RISCV test cases.
This patch also does improve test cases in future patches that will
introduce a binary analysis for llvm-bolt-binary-analysis that checks
for correct application of pac-ret (pointer authentication on return
addresses).
Identical Code Folding (ICF) folds functions that are identical into one
function, and updates symbol addresses to the new address. This reduces
the size of a binary, but can lead to problems. For example when
function pointers are compared. This can be done either explicitly in
the code or generated IR by optimization passes like Indirect Call
Promotion (ICP). After ICF what used to be two different addresses
become the same address. This can lead to a different code path being
taken.
This is where safe ICF comes in. Linker (LLD) does it using address
significant section generated by clang. If symbol is in it, or an object
doesn't have this section symbols are not folded.
BOLT does not have the information regarding which objects do not have
this section, so can't re-use this mechanism.
This implementation scans code section and conservatively marks
functions symbols as unsafe. It treats symbols as unsafe if they are
used in non-control flow instruction. It also scans through the data
relocation sections and does the same for relocations that reference a
function symbol. The latter handles the case when function pointer is
stored in a local or global variable, etc. If a relocation address
points within a vtable these symbols are skipped.
The current implementation of `lookupStubFromGroup` is incorrect. The
function is intended to find and return the closest stub using
`lower_bound`, which identifies the first element in a sorted range that
is not less than a specified value. However, if such an element is not
found within `Candidates` and the list is not empty, the function
returns `nullptr`. Instead, it should check whether the last element
satisfies the condition.
Added support to BOLT for DW_OP_GNU_push_tls_address. So now
DW_TAG_variable with this OP in DW_AT_location will appear in debug
names acceleration table. Although not in the DWARF 5 spec it is similar
to DW_OP_form_tls_address. Without this support llvm-dwarfdump --verify
--debug-names will report errors.
This change affects non-relocation mode only. Prior to having
CheckLargeFunctions pass, we could have emitted code for functions that
was discarded at the end due to size limitations. Since we didn't know
at the time of emission if the code would be discarded or not, we had to
emit jump tables in separate sections and handle them separately.
However, now we always run CheckLargeFunctions and make sure all emitted
code is used. Thus, we can get rid of the special jump table handling.
This fix handles a case where a DIE that does not have
DW_AT_name/DW_AT_linkage_name, but has a reference to another DIE using
DW_AT_abstract_origin/DW_AT_specification. It also fixes a bug where
there are cross CU references for those attributes. Previously it would
use a DWARF Unit of a DIE which was being processed The
warf5-debug-names-cross-cu.s test just happened to work because how it
was constructed where string section was shared by both DWARF Units.
To resolve DW_AT_name/DW_AT_linkage_name this patch iterates over
references until it either reaches the final DIE or finds both of those
names.
Use SymbolStringPtr for Symbol names in LinkGraph. This reduces string interning
on the boundary between JITLink and ORC, and allows pointer comparisons (rather
than string comparisons) between Symbol names. This should improve the
performance and readability of code that bridges between JITLink and ORC (e.g.
ObjectLinkingLayer and ObjectLinkingLayer::Plugins).
To enable use of SymbolStringPtr a std::shared_ptr<SymbolStringPool> is added to
LinkGraph and threaded through to its construction sites in LLVM and Bolt. All
LinkGraphs that are to have symbol names compared by pointer equality must point
to the same SymbolStringPool instance, which in ORC sessions should be the pool
attached to the ExecutionSession.
---------
Co-authored-by: Lang Hames <lhames@gmail.com>
When a callee function is closer than 256MB from its call site, LLD
linker can strategically create a short thunk for the function with a
single branch instruction (that covers +/-128MB). Detect and convert
such thunks into direct calls in BOLT.
When a binary has multiple text segments, the Size is computed as the
difference of the last address of these segments from the BaseAddress.
The base addresses of all text segments must be the same.
Introduces flag 'perf-script-events' for testing, which allows passing
perf events without BOLT having to parse them by invoking 'perf script'.
The flag is used to pass a mock perf profile that has two memory
mappings for a mock binary that has two text segments. The mapping
size is updated as `parseMMapEvents` now processes all text segments.
_init is used during startup of binaires. Unfortunately, its
address can be shared (at least on AArch64 glibc static binaries) with a
data
reference that lives in the GOT. The GOT rewriting is currently unable
to distinguish between data addresses and function addresses. This leads
to the data address being incorrectly rewritten, causing a crash on
startup of the binary:
Unexpected reloc type in static binary.
To avoid this, don't consider _init for being moved, by skipping it.
~We could add further conditions to narrow the skipped case for known
crashes, but as a straw man I thought it'd be best to keep the condition
as simple as possible and see if there any objections to this.~
(Edit: this broke the test
bolt/test/runtime/X86/retpoline-synthetic.test,
because _init was skipped from the retpoline pass and it has an indirect
call in it, so I include a check for static binaries now, which avoids
the test failure,
but perhaps this could/should be narrowed further?)
For now, skip _init for static binaries on any architecture; we could
add further conditions to narrow the skipped case for known crashes, but
as a straw man I thought it'd be best to keep the condition as simple as
possible and see if there any objections to this.
Updates #100096.
This change extracts the comparator for sorting functions by index into
a helper function `compareBinaryFunctionByIndex()`
Not sure why the comparator used in
`BinaryContext::getSortedFunctions()` is not same as the other two
places. I think they should use the same comparator, so I also change
`BinaryContext::getSortedFunctions()` to use
`compareBinaryFunctionByIndex()` for sorting functions.
This caused test failures, see comment on the PR:
Failed Tests (2):
BOLT-Unit :: Core/./CoreTests/AArch64/MemoryMapsTester/MultipleSegmentsMismatchedBaseAddress/0
BOLT-Unit :: Core/./CoreTests/X86/MemoryMapsTester/MultipleSegmentsMismatchedBaseAddress/0
> When a binary has multiple text segments, the Size is computed as the
> difference of the last address of these segments from the BaseAddress.
> The base addresses of all text segments must be the same.
>
> Introduces flag 'perf-script-events' for testing. It allows passing perf events
> without BOLT having to parse them using 'perf script'. The flag is used to
> pass a mock perf profile that has two memory mappings for a mock binary
> that has two text segments. The size of the mapping is updated as this
> change `parseMMapEvents` processes all text segments.
This reverts commit 4b71b3782d217db0138b701c4514bd2168ca1659.
When a binary has multiple text segments, the Size is computed as the
difference of the last address of these segments from the BaseAddress.
The base addresses of all text segments must be the same.
Introduces flag 'perf-script-events' for testing. It allows passing perf events
without BOLT having to parse them using 'perf script'. The flag is used to
pass a mock perf profile that has two memory mappings for a mock binary
that has two text segments. The size of the mapping is updated as this
change `parseMMapEvents` processes all text segments.
Use ULEB128 format for emitting LSDAs for fixed-address executables,
similar to what we use for PIEs/DSOs. Main difference is that we don't
use landing pad trampolines when landing pads are not contained in a
single fragment. Instead, we fallback to emitting larger fixed-address
LSDAs, which is still better than adding trampoline instructions.
We used to emit EH trampolines for PIE/DSO whenever a function fragment
contained a landing pad outside of it. However, it is common to have all
landing pads in a cold fragment even when their throwers are in a hot
one.
To reduce the number of trampolines, analyze landing pads for any given
function fragment, and if they all belong to the same (possibly
different) fragment, designate that fragment as a landing pad fragment
for the "thrower" fragment. Later, emit landing pad fragment symbol as
an LPStart for the thrower LSDA.
Use ULEB128 encoding for call sites in PIE/DSO binaries. The encoding
reduces the size of the tables compared to sdata4 and is the default
format used by Clang.
Note that for fixed-address executables we still use absolute addressing
to cover cases where landing pads can reside in different function
fragments.
For testing, we rely on runtime EH tests.
For C++ exception handling, when we write a call site table, we must
avoid emitting 0-value offsets for landing pads unless the call site has
no landing pad. However, 0 can be a real offset from the start of the
FDE if the FDE corresponds to a function fragment that starts with a
landing pad. In such cases, we used to emit a trap instruction at the
start of the fragment to guarantee non-zero LP offset.
To avoid emitting unnecessary trap instructions, we can instead set
LPStart to an offset from the FDE. If we emit it as [FDEStart - 1], then
all real offsets from LPStart in FDE become non-negative.
Under --use-old-text or --strict, we completely rewrite contents of EH
frames and exception tables sections. If new contents of either section
do not exceed the size of the original section, rewrite the section
in-place.
When emitting C++ exception tables (LSDAs), BOLT used to estimate the
size of the tables beforehand. This implementation was necessary as the
assembler/streamer lacked the emitULEB128IntValue() functionality.
As I plan to introduce [u|s]uleb128-encoded exception tables in BOLT,
now is a perfect time to switch to the new API and eliminate the need
to pre-compute the size of the tables.
"Large" functions are functions that are too big to fit into their
original slots after code modifications. CheckLargeFunctions pass is
designed to prevent such functions from emission. Extend this pass to
work with functions with constant islands.
Now that CheckLargeFunctions covers all functions, it guarantees that we
will never see such functions after code emission on all platforms
(previously it was guaranteed on x86 only). Hence, we can get rid of
RewriteInstance extensions that were meant to support "large" functions.
Absolute thunks generated by LLD reference function addresses recorded
as data in code. Since they are generated by the linker, they don't have
relocations associated with them and thus the addresses are left
undetected. Use pattern matching to detect such thunks and handle them
in VeneerElimination pass.
