Merge "DWARF: Compensate for compiler's off-by-one instruction."
diff --git a/compiler/debug/elf_debug_line_writer.h b/compiler/debug/elf_debug_line_writer.h
index 9875c71..8a4508d 100644
--- a/compiler/debug/elf_debug_line_writer.h
+++ b/compiler/debug/elf_debug_line_writer.h
@@ -114,6 +114,40 @@
continue;
}
+ // Compensate for compiler's off-by-one-instruction error.
+ //
+ // The compiler generates stackmap with PC *after* the branch instruction
+ // (because this is the PC which is easier to obtain when unwinding).
+ //
+ // However, the debugger is more clever and it will ask us for line-number
+ // mapping at the location of the branch instruction (since the following
+ // instruction could belong to other line, this is the correct thing to do).
+ //
+ // So we really want to just decrement the PC by one instruction so that the
+ // branch instruction is covered as well. However, we do not know the size
+ // of the previous instruction, and we can not subtract just a fixed amount
+ // (the debugger would trust us that the PC is valid; it might try to set
+ // breakpoint there at some point, and setting breakpoint in mid-instruction
+ // would make the process crash in spectacular way).
+ //
+ // Therefore, we say that the PC which the compiler gave us for the stackmap
+ // is the end of its associated address range, and we use the PC from the
+ // previous stack map as the start of the range. This ensures that the PC is
+ // valid and that the branch instruction is covered.
+ //
+ // This ensures we have correct line number mapping at call sites (which is
+ // important for backtraces), but there is nothing we can do for non-call
+ // sites (so stepping through optimized code in debugger is not possible).
+ //
+ // We do not adjust the stackmaps if the code was compiled as debuggable.
+ // In that case, the stackmaps should accurately cover all instructions.
+ if (!mi->is_native_debuggable) {
+ for (size_t i = pc2dex_map.size() - 1; i > 0; --i) {
+ pc2dex_map[i].from_ = pc2dex_map[i - 1].from_;
+ }
+ pc2dex_map[0].from_ = 0;
+ }
+
Elf_Addr method_address = base_address + mi->code_address;
PositionInfos dex2line_map;
