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DebuggingTheCompiler.rst

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Debugging the Swift Compiler

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Abstract

This document contains some useful information for debugging the swift compiler.

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 oprimization 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 afer 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 on SIL Level

Options for Dumping the SIL

Often it is not sufficient to dump the SIL at the begin 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 SILPassManager.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.

Using Breakpoints

LLDB has very powerful breakpoints, which can be utilized in many ways to debug the compiler. 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::create and check for the function name in the breakpoint condition:

(lldb) br set -c 'name.equals("_TFC3nix1Xd")' -n SILFunction::create

Sometimes you want to know which optimization does insert, remove or move 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 &&
           I->getFunction()->getName().equals("_TFC3nix1Xd")'
           -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
> DONE

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.