1 files changed, 74 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_gcc/doc/debugging-libgccjit.md b/compiler/rustc_codegen_gcc/doc/debugging-libgccjit.md
new file mode 100644
index 00000000000..be0ec83f7cd
--- /dev/null
+++ b/compiler/rustc_codegen_gcc/doc/debugging-libgccjit.md
@@ -0,0 +1,74 @@
+# Debugging libgccjit
+
+Sometimes, libgccjit will crash and output an error like this:
+
+```
+during RTL pass: expand
+libgccjit.so: error: in expmed_mode_index, at expmed.h:249
+0x7f0da2e61a35 expmed_mode_index
+	../../../gcc/gcc/expmed.h:249
+0x7f0da2e61aa4 expmed_op_cost_ptr
+	../../../gcc/gcc/expmed.h:271
+0x7f0da2e620dc sdiv_cost_ptr
+	../../../gcc/gcc/expmed.h:540
+0x7f0da2e62129 sdiv_cost
+	../../../gcc/gcc/expmed.h:558
+0x7f0da2e73c12 expand_divmod(int, tree_code, machine_mode, rtx_def*, rtx_def*, rtx_def*, int)
+	../../../gcc/gcc/expmed.c:4335
+0x7f0da2ea1423 expand_expr_real_2(separate_ops*, rtx_def*, machine_mode, expand_modifier)
+	../../../gcc/gcc/expr.c:9240
+0x7f0da2cd1a1e expand_gimple_stmt_1
+	../../../gcc/gcc/cfgexpand.c:3796
+0x7f0da2cd1c30 expand_gimple_stmt
+	../../../gcc/gcc/cfgexpand.c:3857
+0x7f0da2cd90a9 expand_gimple_basic_block
+	../../../gcc/gcc/cfgexpand.c:5898
+0x7f0da2cdade8 execute
+	../../../gcc/gcc/cfgexpand.c:6582
+```
+
+To see the code which causes this error, call the following function:
+
+```c
+gcc_jit_context_dump_to_file(ctxt, "/tmp/output.c", 1 /* update_locations */)
+```
+
+This will create a C-like file and add the locations into the IR pointing to this C file.
+Then, rerun the program and it will output the location in the second line:
+
+```
+libgccjit.so: /tmp/something.c:61322:0: error: in expmed_mode_index, at expmed.h:249
+```
+
+Or add a breakpoint to `add_error` in gdb and print the line number using:
+
+```
+p loc->m_line
+p loc->m_filename->m_buffer
+```
+
+To print a debug representation of a tree:
+
+```c
+debug_tree(expr);
+```
+
+(defined in print-tree.h)
+
+To print a debug representation of a gimple struct:
+
+```c
+debug_gimple_stmt(gimple_struct)
+```
+
+To get the `rustc` command to run in `gdb`, add the `--verbose` flag to `cargo build`.
+
+To have the correct file paths in `gdb` instead of `/usr/src/debug/gcc/libstdc++-v3/libsupc++/eh_personality.cc`:
+
+Maybe by calling the following at the beginning of gdb:
+
+```
+set substitute-path /usr/src/debug/gcc /path/to/gcc-repo/gcc
+```
+
+TODO(antoyo): but that's not what I remember I was doing.