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Debugging the Swift Compiler


This document contains some useful information for debugging the Swift compiler and Swift compiler output.

Basic Utilities

Often, the first step to debug a compiler problem is to re-run the compiler with a command line, which comes from a crash trace or a build log.

The script split-cmdline in utils/dev-scripts splits a command line into multiple lines. This is helpful to understand and edit long command lines.

Printing the Intermediate Representations

The most important thing when debugging the compiler is to examine the IR. Here is how to dump the IR after the main phases of the Swift compiler (assuming you are compiling with optimizations enabled):

  1. Parser. To print the AST after parsing:

    swiftc -dump-ast -O file.swift
  2. SILGen. To print the SIL immediately after SILGen:

    swiftc -emit-silgen -O file.swift
  3. Mandatory SIL passes. To print the SIL after the mandatory passes:

    swiftc -emit-sil -Onone file.swift
Well, this is not quite true, because the compiler is running some passes for -Onone after the mandatory passes, too. But for most purposes you will get what you want to see.
  1. Performance SIL passes. To print the SIL after the complete SIL optimization pipeline:

    swiftc -emit-sil -O file.swift
  2. IRGen. To print the LLVM IR after IR generation:

    swiftc -emit-ir -Xfrontend -disable-llvm-optzns -O file.swift
  1. LLVM passes. To print the LLVM IR after LLVM passes:

    swiftc -emit-ir -O file.swift
  2. Code generation. To print the final generated code:

    swiftc -S -O file.swift

Compilation stops at the phase where you print the output. So if you want to print the SIL and the LLVM IR, you have to run the compiler twice. The output of all these dump options (except -dump-ast) can be redirected with an additional -o <file> option.

Debugging the Type Checker

Enabling Logging

To enable logging in the type checker, use the following argument: -Xfrontend -debug-constraints. This will cause the typechecker to log its internal state as it solves constraints and present the final type checked solution, e.g.:

---Constraint solving for the expression at [test.swift:3:10 - line:3:10]---
---Initial constraints for the given expression---
(integer_literal_expr type='$T0' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] value=0)
Score: 0 0 0 0 0 0 0 0 0 0 0 0 0
Contextual Type: Int
Type Variables:
  #0 = $T0 [inout allowed]

Active Constraints:

Inactive Constraints:
  $T0 literal conforms to ExpressibleByIntegerLiteral [[locator@0x7ffa3a865a00 [IntegerLiteral@test.swift:3:10]]];
  $T0 conv Int [[locator@0x7ffa3a865a00 [IntegerLiteral@test.swift:3:10]]];
($T0 literal=3 bindings=(subtypes of) (default from ExpressibleByIntegerLiteral) Int)
Active bindings: $T0 := Int
(trying $T0 := Int
  (found solution 0 0 0 0 0 0 0 0 0 0 0 0 0)
Fixed score: 0 0 0 0 0 0 0 0 0 0 0 0 0
Type variables:
  $T0 as Int

Overload choices:

Constraint restrictions:

Disjunction choices:

  At locator@0x7ffa3a865a00 [IntegerLiteral@test.swift:3:10]
(normal_conformance type=Int protocol=ExpressibleByIntegerLiteral lazy
  (normal_conformance type=Int protocol=_ExpressibleByBuiltinIntegerLiteral lazy))
(found solution 0 0 0 0 0 0 0 0 0 0 0 0 0)
---Type-checked expression---
(call_expr implicit type='Int' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] arg_labels=_builtinIntegerLiteral:
  (constructor_ref_call_expr implicit type='(_MaxBuiltinIntegerType) -> Int' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10]
    (declref_expr implicit type='(Int.Type) -> (_MaxBuiltinIntegerType) -> Int' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] decl=Swift.(file).Int.init(_builtinIntegerLiteral:) function_ref=single)
    (type_expr implicit type='Int.Type' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] typerepr='Int'))
  (tuple_expr implicit type='(_builtinIntegerLiteral: Int2048)' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] names=_builtinIntegerLiteral
    (integer_literal_expr type='Int2048' location=test.swift:3:10 range=[test.swift:3:10 - line:3:10] value=0)))

When using the integrated swift-repl, one can dump the same output for each expression as one evaluates the expression by enabling constraints debugging by typing :constraints debug on:

$ swift -frontend -repl -enable-objc-interop -module-name REPL
***  You are running Swift's integrated REPL,  ***
***  intended for compiler and stdlib          ***
***  development and testing purposes only.    ***
***  The full REPL is built as part of LLDB.   ***
***  Type ':help' for assistance.              ***
(swift) :constraints debug on

Asserting on First Error

When changing the typechecker, one can cause a series of cascading errors. Since Swift doesn't assert on such errors, one has to know more about the typechecker to know where to stop in the debugger. Rather than doing that, one can use the option -Xllvm -swift-diagnostics-assert-on-error=1 to cause the DiagnosticsEngine to assert upon the first error, providing the signal that the debugger needs to know that it should attach.

Debugging on SIL Level

Options for Dumping the SIL

Often it is not sufficient to dump the SIL at the beginning or end of the optimization pipeline. The SILPassManager supports useful options to dump the SIL also between pass runs.

The option -Xllvm -sil-print-all dumps the whole SIL module after all passes. Although it prints only functions which were changed by a pass, the output can get very large.

It is useful if you identified a problem in the final SIL and you want to check which pass did introduce the wrong SIL.

There are several other options available, e.g. to filter the output by function names (-Xllvm -sil-print-only-function/s) or by pass names (-Xllvm -sil-print-before/after/around). For details see PassManager.cpp.

Dumping the SIL and other Data in LLDB

When debugging the Swift compiler with LLDB (or Xcode, of course), there is even a more powerful way to examine the data in the compiler, e.g. the SIL. Following LLVM's dump() convention, many SIL classes (as well as AST classes) provide a dump() function. You can call the dump function with LLDB's expression -- or print or p command.

For example, to examine a SIL instruction:

(lldb) p Inst->dump()
%12 = struct_extract %10 : $UnsafeMutablePointer<X>, #UnsafeMutablePointer._rawValue // user: %13

To dump a whole function at the beginning of a function pass:

(lldb) p getFunction()->dump()

SIL modules and even functions can get very large. Often it is more convenient to dump their contents into a file and open the file in a separate editor. This can be done with:

(lldb) p getFunction()->dump("myfunction.sil")

You can also dump the CFG (control flow graph) of a function:

(lldb) p Func->viewCFG()

This opens a preview window containing the CFG of the function. To continue debugging press <CTRL>-C on the LLDB prompt. Note that this only works in Xcode if the PATH variable in the scheme's environment setting contains the path to the dot tool.

Debugging and Profiling on SIL level

The compiler provides a way to debug and profile on SIL level. To enable SIL debugging add the front-end option -gsil together with -g. Example:

swiftc -g -Xfrontend -gsil -O test.swift -o a.out

This writes the SIL after optimizations into a file and generates debug info for it. In the debugger and profiler you can then see the SIL code instead of the Swift source code. For details see the SILDebugInfoGenerator pass.

To enable SIL debugging and profiling for the Swift standard library, use the build-script-impl option --build-sil-debugging-stdlib.

ViewCFG: Regex based CFG Printer

ViewCFG (./utils/viewcfg) is a script that parses a textual CFG (e.g. a llvm or sil function) and displays a .dot file of the CFG. Since the parsing is done using regular expressions (i.e. ignoring language semantics), ViewCFG can:

  1. Parse both SIL and LLVM IR
  2. Parse blocks and functions without needing to know contextual information. Ex: types and declarations.

The script assumes that the relevant text is passed in via stdin and uses open to display the .dot file.

Additional, both emacs and vim integration is provided. For vim integration add the following commands to your .vimrc:

com! -nargs=? Funccfg silent ?{$?,/^}/w !viewcfg <args>
com! -range -nargs=? Viewcfg silent <line1>,<line2>w !viewcfg <args>

This will add:

:Funccfg        displays the CFG of the current SIL/LLVM function.
:<range>Viewcfg displays the sub-CFG of the selected range.

For emacs users, we provide in sil-mode (./utils/sil-mode.el) the function:


To use this feature, placed the point in the sil function that you want viewcfg to graph and then run sil-mode-display-function-cfg. This will cause viewcfg to be invoked with the sil function body. Note, sil-mode-display-function-cfg does not take any arguments.

NOTE viewcfg must be in the $PATH for viewcfg to work.

NOTE Since we use open, .dot files should be associated with the Graphviz app for viewcfg to work.

There is another useful script to view the CFG of a disassembled function: ./utils/dev-scripts/blockifyasm. It splits a disassembled function up into basic blocks which can then be used with viewcfg:

(lldb) disassemble
  <copy-paste output to file.s>
$ blockifyasm < file.s | viewcfg

Using Breakpoints

LLDB has very powerful breakpoints, which can be utilized in many ways to debug the compiler and Swift executables. The examples in this section show the LLDB command lines. In Xcode you can set the breakpoint properties by clicking 'Edit breakpoint'.

Let's start with a simple example: sometimes you see a function in the SIL output and you want to know where the function was created in the compiler. In this case you can set a conditional breakpoint in SILFunction constructor and check for the function name in the breakpoint condition:

(lldb) br set -c 'hasName("_TFC3nix1Xd")' -f SILFunction.cpp -l 91

Sometimes you may want to know which optimization inserts, removes or moves a certain instruction. To find out, set a breakpoint in ilist_traits<SILInstruction>::addNodeToList or ilist_traits<SILInstruction>::removeNodeFromList, which are defined in SILInstruction.cpp. The following command sets a breakpoint which stops if a strong_retain instruction is removed:

(lldb) br set -c 'I->getKind() == ValueKind::StrongRetainInst' -f SILInstruction.cpp -l 63

The condition can be made more precise e.g. by also testing in which function this happens:

(lldb) br set -c 'I->getKind() == ValueKind::StrongRetainInst &&
           -f SILInstruction.cpp -l 63

Let's assume the breakpoint hits somewhere in the middle of compiling a large file. This is the point where the problem appears. But often you want to break a little bit earlier, e.g. at the entrance of the optimization's run function.

To achieve this, set another breakpoint and add breakpoint commands:

(lldb) br set -n GlobalARCOpts::run
Breakpoint 2
(lldb) br com add 2
> p int $n = $n + 1
> c

Run the program (this can take quite a bit longer than before). When the first breakpoint hits see what value $n has:

(lldb) p $n
(int) $n = 5

Now remove the breakpoint commands from the second breakpoint (or create a new one) and set the ignore count to $n minus one:

(lldb) br delete 2
(lldb) br set -i 4 -n GlobalARCOpts::run

Run your program again and the breakpoint hits just before the first breakpoint.

Another method for accomplishing the same task is to set the ignore count of the breakpoint to a large number, i.e.:

(lldb) br set -i 9999999 -n GlobalARCOpts::run

Then whenever the debugger stops next time (due to hitting another breakpoint/crash/assert) you can list the current breakpoints:

(lldb) br list
1: name = 'GlobalARCOpts::run', locations = 1, resolved = 1, hit count = 85 Options: ignore: 1 enabled

which will then show you the number of times that each breakpoint was hit. In this case, we know that GlobalARCOpts::run was hit 85 times. So, now we know to ignore swift_getGenericMetadata 84 times, i.e.:

(lldb) br set -i 84 -n GlobalARCOpts::run

LLDB Scripts

LLDB has powerful capabilities of scripting in Python among other languages. An often overlooked, but very useful technique is the -s command to lldb. This essentially acts as a pseudo-stdin of commands that lldb will read commands from. Each time lldb hits a stopping point (i.e. a breakpoint or a crash/assert), it will run the earliest command that has not been run yet. As an example of this consider the following script (which without any loss of generality will be called test.lldb):

env DYLD_INSERT_LIBRARIES=/usr/lib/libgmalloc.dylib
break set -n swift_getGenericMetadata
break mod 1 -i 83
process launch -- --stdlib-unittest-in-process --stdlib-unittest-filter "DefaultedForwardMutableCollection<OpaqueValue<Int>>.Type.subscript(_: Range)/Set/semantics"
break set -l 224
expr pattern->CreateFunction
break set -a $0
dis -f

TODO: Change this example to apply to the Swift compiler instead of to the stdlib unittests.

Then by running lldb test -s test.lldb, lldb will:

  1. Enable guard malloc.
  2. Set a break point on swift_getGenericMetadata and set it to be ignored for 83 hits.
  3. Launch the application and stop at swift_getGenericMetadata after 83 hits have been ignored.
  4. In the same file as swift_getGenericMetadata introduce a new breakpoint at line 224 and continue.
  5. When we break at line 224 in that file, evaluate an expression pointer.
  6. Set a breakpoint at the address of the expression pointer and continue.
  7. When we hit the breakpoint set at the function pointer's address, disassemble the function that the function pointer was passed to.

Using LLDB scripts can enable one to use complex debugger workflows without needing to retype the various commands perfectly every time.

Custom LLDB Commands

If you've ever found yourself repeatedly entering a complex sequence of commands within a debug session, consider using custom lldb commands. Custom commands are a handy way to automate debugging tasks.

For example, say we need a command that prints the contents of the register rax and then steps to the next instruction. Here's how to define that command within a debug session:

(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> def custom_step():
...   print "rax =", lldb.frame.FindRegister("rax")
...   lldb.thread.StepInstruction(True)
>>> ^D

You can call this function using the script command, or via an alias:

(lldb) script custom_step()
rax = ...
<debugger steps to the next instruction>

(lldb) command alias cs script custom_step()
(lldb) cs
rax = ...
<debugger steps to the next instruction>

Printing registers and single-stepping are by no means the only things you can do with custom commands. The LLDB Python API surfaces a lot of useful functionality, such as arbitrary expression evaluation.

There are some pre-defined custom commands which can be especially useful while debugging the swift compiler. These commands live in swift/utils/lldb/ There is a wrapper script available in SWIFT_BINARY_DIR/bin/lldb-with-tools which launches lldb with those commands loaded.

A command named sequence is included in lldbToolBox. sequence runs multiple semicolon separated commands together as one command. This can be used to define custom commands using just other lldb commands. For example, custom_step() function defined above could be defined as:

(lldb) command alias cs sequence p/x $rax; stepi

Reducing SIL test cases using bug_reducer

There is functionality provided in ./swift/utils/bug_reducer/ for reducing SIL test cases by:

  1. Producing intermediate sib files that only require some of the passes to trigger the crasher.
  2. Reducing the size of the sil test case by extracting functions or partitioning a module into unoptimized and optimized modules.

For more information and a high level example, see: ./swift/utils/bug_reducer/

Using clang-tidy to run the Static Analyzer

Recent versions of LLVM package the tool clang-tidy. This can be used in combination with a json compilation database to run static analyzer checks as well as cleanups/modernizations on a code-base. Swift's cmake invocation by default creates one of these json databases at the root path of the swift host build, for example on macOS:


Using this file, one invokes clang-tidy on a specific file in the codebase as follows:

clang-tidy -p=$PATH_TO_BUILD/swift-macosx-x86_64/compile_commands.json $FULL_PATH_TO_FILE

One can also use shell regex to visit multiple files in the same directory. Example:

clang-tidy -p=$PATH_TO_BUILD/swift-macosx-x86_64/compile_commands.json $FULL_PATH_TO_DIR/*.cpp

Debugging Swift Executables

One can use the previous tips for debugging the Swift compiler with Swift executables as well. Here are some additional useful techniques that one can use in Swift executables.

Determining the mangled name of a function in LLDB

One problem that often comes up when debugging Swift code in LLDB is that LLDB shows the demangled name instead of the mangled name. This can lead to mistakes where due to the length of the mangled names one will look at the wrong function. Using the following command, one can find the mangled name of the function in the current frame:

(lldb) image lookup -va $pc
Address: CollectionType3[0x0000000100004db0] (CollectionType3.__TEXT.__text + 16000)
Summary: CollectionType3`ext.CollectionType3.CollectionType3.MutableCollectionType2<A where A: CollectionType3.MutableCollectionType2>.(subscript.materializeForSet : (Swift.Range<A.Index>) -> Swift.MutableSlice<A>).(closure #1)
Module: file = "/Volumes/Files/work/solon/build/build-swift/validation-test-macosx-x86_64/stdlib/Output/CollectionType.swift.gyb.tmp/CollectionType3", arch = "x86_64"
Symbol: id = {0x0000008c}, range = [0x0000000100004db0-0x00000001000056f0), name="ext.CollectionType3.CollectionType3.MutableCollectionType2<A where A: CollectionType3.MutableCollectionType2>.(subscript.materializeForSet : (Swift.Range<A.Index>) -> Swift.MutableSlice<A>).(closure #1)", mangled="_TFFeRq_15CollectionType322MutableCollectionType2_S_S0_m9subscriptFGVs5Rangeqq_s16MutableIndexable5Index_GVs12MutableSliceq__U_FTBpRBBRQPS0_MS4__T_"

Manually symbolication using LLDB

One can perform manual symbolication of a crash log or an executable using LLDB without running the actual executable. For a detailed guide on how to do this, see:

Debugging LLDB failures

Sometimes one needs to be able to while debugging actually debug LLDB and its interaction with Swift itself. Some examples of problems where this can come up are:

  1. Compiler bugs when LLDB attempts to evaluate an expression. (expression debugging)
  2. Swift variables being shown with no types. (type debugging)

To gain further insight into these sorts of failures, we use LLDB log categories. LLDB log categories provide introspection by causing LLDB to dump verbose information relevant to the category into the log as it works. The two log channels that are useful for debugging Swift issues are the "types" and "expression" log channels.

For more details about any of the information below, please run:

(lldb) help log enable

"Types" Log

The "types" log reports on LLDB's process of constructing SwiftASTContexts and errors that may occur. The two main tasks here are:

  1. Constructing the SwiftASTContext for a specific single Swift module. This is used to implement frame local variable dumping via the lldb frame variable command, as well as the Xcode locals view. On failure, local variables will not have types.
  2. Building a SwiftASTContext in which to run Swift expressions using the "expression" command. Upon failure, one will see an error like: "Shared Swift state for has developed fatal errors and is being discarded."

These errors can be debugged by turning on the types log:

(lldb) log enable -f /tmp/lldb-types-log.txt lldb types

That will write the types log to the file passed to the -f option.

NOTE Module loading can happen as a side-effect of other operations in lldb

(e.g. the "file" command). To be sure that one has enabled logging before /any/ module loading has occurred, place the command into either:


This will ensure that the type import command is run before /any/ modules are imported.

"Expression" Log

The "expression" log reports on the process of wrapping, parsing, SILGen'ing, JITing, and inserting an expression into the current Swift module. Since this can only be triggered by the user manually evaluating expression, this can be turned on at any point before evaluating an expression. To enable expression logging, first run:

(lldb) log enable -f /tmp/lldb-expr-log.txt lldb expression

and then evaluate the expression. The expression log dumps, in order, the following non-exhaustive list of state:

  1. The unparsed, textual expression passed to the compiler.
  2. The parsed expression.
  3. The initial SILGen.
  4. SILGen after SILLinking has occurred.
  5. SILGen after SILLinking and Guaranteed Optimizations have occurred.
  6. The resulting LLVM IR.
  7. The assembly code that will be used by the JIT.

NOTE LLDB runs a handful of preparatory expressions that it uses to set up for running Swift expressions. These can make the expression logs hard to read especially if one evaluates multiple expressions with the logging enabled. In such a situation, run all expressions before the bad expression, turn on the logging, and only then run the bad expression.

Multiple Logs at a Time

Note, you can also turn on more than one log at a time as well, e.x.:

(lldb) log enable -f /tmp/lldb-types-log.txt lldb types expression