cmd/ld: fix off-by-one error in DWARF .debug_line transcription
The liblink refactor changed the DWARF .debug_line flow control. The mapping was off by one pcline entry. The fix here preserves pc until it can be compared to pcline.pc.
Sample dwarfdump .debug_line output for main.main from the program in issue 7351, before liblink (correct):
0x0000003c: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00000047: 03 DW_LNS_advance_line( 6 )
0x00000049: 01 DW_LNS_copy
0x0000000000002000 1 7 0 is_stmt
0x0000004a: 8b address += 21, line += 1
0x0000000000002021 1 8 0 is_stmt
0x0000004b: 02 DW_LNS_advance_pc( 153 )
0x0000004e: 03 DW_LNS_advance_line( 1 )
0x00000050: 01 DW_LNS_copy
0x00000000000020ba 1 9 0 is_stmt
After liblink (off by one entry):
0x00001bbf: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00001bca: 02 DW_LNS_advance_pc( 33 )
0x00001bcc: 03 DW_LNS_advance_line( 6 )
0x00001bce: 01 DW_LNS_copy
0x0000000000002021 1 7 0 is_stmt
0x00001bcf: 02 DW_LNS_advance_pc( 153 )
0x00001bd2: 03 DW_LNS_advance_line( 1 )
0x00001bd4: 01 DW_LNS_copy
0x00000000000020ba 1 8 0 is_stmt
0x00001bd5: 02 DW_LNS_advance_pc( 153 )
0x00001bd8: 03 DW_LNS_advance_line( 1 )
0x00001bda: 01 DW_LNS_copy
0x0000000000002153 1 9 0 is_stmt
After this CL (the line 9 pc offset changed due to intervening compiler changes):
0x00001d07: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00001d12: 03 DW_LNS_advance_line( 6 )
0x00001d14: 01 DW_LNS_copy
0x0000000000002000 1 7 0 is_stmt
0x00001d15: 8b address += 21, line += 1
0x0000000000002021 1 8 0 is_stmt
0x00001d16: 02 DW_LNS_advance_pc( 189 )
0x00001d19: 03 DW_LNS_advance_line( 1 )
0x00001d1b: 01 DW_LNS_copy
0x00000000000020de 1 9 0 is_stmt
Fixes #7351.
LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/
66290043