Lines Matching full:debug
1 # Debug info migration: From intrinsics to records
3 We're planning on removing debug info intrinsics from LLVM, as they're slow, unwieldy and can confuse optimisation passes if they're not expecting them. Instead of having a sequence of instructions that looks like this:
13 with `dbg.value` intrinsics representing debug info records, it would instead be printed as:
23 The debug records are not instructions, do not appear in the instruction list, and won't appear in your optimisation passes unless you go digging for them deliberately.
33 Then you must past that iterator into the insertion function without modification (the iterator carries a debug-info bit). That's all! Read on for a more detailed explanation.
37 There are two significant changes to be aware of. Firstly, we're adding a single bit of debug relevant data to the `BasicBlock::iterator` class (it's so that we can determine whether ranges intend on including debug info at the beginning of a block or not). That means when writing passes that insert LLVM IR instructions, you need to identify positions with `BasicBlock::iterator` rather than just a bare `Instruction *`. Most of the time this means that after identifying where you intend on inserting something, you must also call `getIterator` on the instruction position -- however when inserting at the start of a block you _must_ use `getFirstInsertionPt`, `getFirstNonPHIIt` or `begin` and use that iterator to insert, rather than just fetching a pointer to the first instruction.
39 The second matter is that if you transfer sequences of instructions from one place to another manually, i.e. repeatedly using `moveBefore` where you might have used `splice`, then you should instead use the method `moveBeforePreserving`. `moveBeforePreserving` will transfer debug info records with the instruction they're attached to. This is something that happens automatically today -- if you use `moveBefore` on every element of an instruction sequence, then debug intrinsics will be moved in the normal course of your code, but we lose this behaviour with non-instruction debug info.
41 For a more in-depth overview of how to update existing code to support debug records, see [the guide below](#how-to-update-existing-code).
45 As we change from using debug intrinsics to debug records, any tools that depend on parsing IR produced by LLVM will need to handle the new format. For the most part, the difference between the printed form of a debug intrinsic call and a debug record is trivial:
50 4. The DILocation changes from being an instruction attachment with the format `!dbg !<Num>`, to being an ordinary argument, i.e. `!<Num>`, that is passed as the final argument to the debug record.
52 Following these rules, we have this example of a debug intrinsic and the equivalent debug record:
55 ; Debug Intrinsic:
57 ; Debug Record:
101 # Any test that explicitly tests debug intrinsic output is not suitable to
133 LLVM :: Transforms/HotColdSplit/transfer-debug-info.ll
136 LLVM :: Transforms/SafeStack/X86/debug-loc2.ll
151 LLVMDIBuilderInsertDeclareBefore # Insert a debug record (new debug info format) instead of a debug intrinsic (old debug info format).
159 LLVMSetIsNewDbgInfoFormat # Convert to the requested debug info format.
163 LLVMGetFirstDbgRecord # Obtain the first debug record attached to an instruction.
164 LLVMGetLastDbgRecord # Obtain the last debug record attached to an instruction.
165 LLVMGetNextDbgRecord # Get next debug record or NULL.
166 LLVMGetPreviousDbgRecord # Get previous debug record or NULL.
167 LLVMDIBuilderInsertDeclareRecordBefore # Insert a debug record (new debug info format).
178 `LLVMPositionBuilderBeforeDbgRecords` and `LLVMPositionBuilderBeforeInstrAndDbgRecords` behave the same as `LLVMPositionBuilder` and `LLVMPositionBuilderBefore` except the insertion position is set before the debug records that precede the target instruction. Note that this doesn't mean that debug intrinsics before the chosen instruction are skipped, only debug records (which unlike debug records are not themselves instructions).
181 If you are trying to insert at the start of a block, or purposfully skip debug intrinsics to determine the insertion point for any other reason, then call the new functions.
183 `LLVMPositionBuilder` and `LLVMPositionBuilderBefore` are unchanged. They insert before the indicated instruction but after any attached debug records.
185 `LLVMGetFirstDbgRecord`, `LLVMGetLastDbgRecord`, `LLVMGetNextDbgRecord` and `LLVMGetPreviousDbgRecord` can be used for iterating over debug records attached to instructions (provided as `LLVMValueRef`).
203 # The new "Debug Record" model
205 Below is a brief overview of the new representation that replaces debug intrinsics; for an instructive guide on updating old code, see [here](#how-to-update-existing-code).
207 ## What exactly have you replaced debug intrinsics with?
209 We're using a dedicated C++ class called `DbgRecord` to store debug info, with a one-to-one relationship between each instance of a debug intrinsic and each `DbgRecord` object in any LLVM IR program; these `DbgRecord`s are represented in the IR as non-instruction debug records, as described in the [Source Level Debugging](project:SourceLevelDebugging.rst#Debug Records) document. This class has a set of subclasses that store exactly the same information as is stored in debugging intrinsics. Each one also has almost entirely the same set of methods, that behave in the same way:
245 The various kinds of debug intrinsic (value, declare, assign, label) are all stored in `DbgRecord` subclasses, with a "RecordKind" field distinguishing `DbgLabelRecord`s from `DbgVariableRecord`s, and a `LocationType` field in the `DbgVariableRecord` class further disambiguating the various debug variable intrinsics it can represent.
249 Any existing code that interacts with debug intrinsics in some way will need to be updated to interact with debug records in the same way. A few quick rules to keep in mind when updating code:
251 - Debug records will not be seen when iterating over instructions; to find the debug records that appear immediately before an instruction, you'll need to iterate over `Instruction::getDbgRecordRange()`.
252 - Debug records have interfaces that are identical to those of debug intrinsics, meaning that any code that operates on debug intrinsics can be trivially applied to debug records as well. The exceptions for this are `Instruction` or `CallInst` methods that don't logically apply to debug records, and `isa`/`cast`/`dyn_cast` methods, are replaced by methods on the `DbgRecord` class itself.
253 - Debug records cannot appear in a module that also contains debug intrinsics; the two are mutually exclusive. As debug records are the future format, handling records correctly should be prioritized in new code.
254 - Until support for intrinsics is no longer present, a valid hotfix for code that only handles debug intrinsics and is non-trivial to update is to convert the module to the intrinsic format using `Module::setIsNewDbgInfoFormat`, and convert it back afterwards.
262 // Module returns to previous debug info format after exiting the above block.
266 Below is a rough guide on how existing code that currently supports debug intrinsics can be updated to support debug records.
268 ## Creating debug records
270 Debug records will automatically be created by the `DIBuilder` class when the new format is enabled. As with instructions, it is also possible to call `DbgRecord::clone` to create an unattached copy of an existing record.
272 ## Skipping debug records, ignoring debug-uses of `Values`, stably counting instructions, etc.
278 // Old: Skips debug intrinsics
281 // New: No extra code needed, debug records are skipped by default.
286 ## Finding debug records
310 ## Examining debug records at positions
316 // Old: Uses a data member of a debug intrinsic, and then skips to the next
322 // New: Iterates over the debug records that appear before `I`, and treats
325 // process the debug records preceding `I` before `I` itself.
349 // Debug variable records are not cast to subclasses; simply call the
358 ## Processing individual debug records
360 In most cases, any code that operates on debug intrinsics can be extracted to a template function or auto lambda (if it is not already in one) that can be applied to both debug intrinsics and debug records - though keep in mind the main exception that `isa`/`cast`/`dyn_cast` do not apply to `DbgVariableRecord` types.
363 // Old: Function that operates on debug variable intrinsics in a BasicBlock, and
412 ## Moving and deleting debug records
419 // Old: Move a debug intrinsic to the start of the block, and delete all other intrinsics for the same variable in the block.
431 // New: Perform the same operation, but for a debug record.
444 ## What about dangling debug records?
455 your optimisation pass may wish to erase the terminator and then do something to the block. This is easy to do when debug info is kept in instructions, but with `DbgRecord`s there is no trailing instruction to attach the variable information to in the block above, once the terminator is erased. For such degenerate blocks, `DbgRecord`s are stored temporarily in a map in `LLVMContext`, and are re-inserted when a terminator is reinserted to the block or other instruction inserted at `end()`.
461 The above guide does not comprehensively cover every pattern that could apply to debug intrinsics; as mentioned at the [start of the guide](#how-to-update-existing-code), you can temporarily convert the target module from debug records to intrinsics as a stopgap measure. Most operations that can be performed on debug intrinsics have exact equivalents for debug records, but if you encounter any exceptions, reading the class docs (linked [here](#what-exactly-have-you-replaced-debug-intrinsics-with)) may give some insight, there may be examples in the existing codebase, and you can always ask for help on the [forums](https://discourse.llvm.org/tag/debuginfo).