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