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[DebuggingTheCompiler] Document a few flags for dumping llvm-ir. #34185

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Oct 6, 2020
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[DebuggingTheCompiler] Document a few flags for dumping llvm-ir. #34185

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16 changes: 16 additions & 0 deletions docs/DebuggingTheCompiler.md
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Expand Up @@ -30,6 +30,8 @@ benefit of all Swift developers.
- [Debugging the Compiler using advanced LLDB Breakpoints](#debugging-the-compiler-using-advanced-lldb-breakpoints)
- [Debugging the Compiler using LLDB Scripts](#debugging-the-compiler-using-lldb-scripts)
- [Custom LLDB Commands](#custom-lldb-commands)
- [Debugging at LLVM Level](#debugging-at-llvm-level)
- [Options for Dumping LLVM IR](#options-for-dumping-llvm-ir)
- [Bisecting Compiler Errors](#bisecting-compiler-errors)
- [Bisecting on SIL optimizer pass counts to identify optimizer bugs](#bisecting-on-sil-optimizer-pass-counts-to-identify-optimizer-bugs)
- [Using git-bisect in the presence of branch forwarding/feature branches](#using-git-bisect-in-the-presence-of-branch-forwardingfeature-branches)
Expand Down Expand Up @@ -537,6 +539,20 @@ to define custom commands using just other lldb commands. For example,

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

## Debugging at LLVM Level

### Options for Dumping LLVM IR

Similar to SIL, one can configure LLVM to dump the llvm-ir at various points in
the pipeline. Here is a quick summary of the various options:

* ``-Xllvm -print-before=$PASS_ID``: Print the LLVM IR before a specified LLVM pass runs.
* ``-Xllvm -print-before-all``: Print the LLVM IR before each pass runs.
* ``-Xllvm -print-after-all``: Print the LLVM IR after each pass runs.
* ``-Xllvm -filter-print-funcs=$FUNC_NAME_1,$FUNC_NAME_2,...,$FUNC_NAME_N``:
When printing IR for functions for print-[before|after]-all options, Only
print the IR for functions whose name is in this comma separated list.

## Bisecting Compiler Errors

### Bisecting on SIL optimizer pass counts to identify optimizer bugs
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