Package: emacs;
Reported by: Alan Mackenzie <acm <at> muc.de>
Date: Tue, 27 Apr 2021 14:50:01 UTC
Severity: normal
Merged with 48100
Found in version 28.0.50
Done: Alan Mackenzie <acm <at> muc.de>
Bug is archived. No further changes may be made.
View this message in rfc822 format
From: Alan Mackenzie <acm <at> muc.de> To: 48061 <at> debbugs.gnu.org Subject: bug#48061: Unexpected result from a native-compiled function Date: Tue, 27 Apr 2021 17:20:22 +0000
On Tue, Apr 27, 2021 at 14:49:31 +0000, Alan Mackenzie wrote:
> Hello, Emacs.
> In certain circumstances (see below for recipe), the natively compiled
> version of c-determine-limit-no-macro returns an invalid result, nil.
> In the same circumstances, the edebug instrumented version returns the
> correct result, a buffer position.
> So far I have tried M-x disassemble RET c-determine-limit-no-macro, but
> I wasn't able to follow the output (there were no symbols in the
> listing).
I've now managed to get a decent disassembly, and there is indeed a
missing machine instruction in the code which causes it to fail:
The function is:
#########################################################################
(defun c-determine-limit-no-macro (here org-start)
;; If HERE is inside a macro, and ORG-START is not also in the same macro,
;; return the beginning of the macro. Otherwise return HERE. Point is not
;; preserved by this function.
(goto-char here)
(let ((here-BOM (and (c-beginning-of-macro) (point))))
(if (and here-BOM
(not (eq (progn (goto-char org-start)
(and (c-beginning-of-macro) (point)))
here-BOM)))
here-BOM
here)))
#########################################################################
The register use in the compiled function is:
rbp here
r12 org-start
r13 here-BOM
The disassembly (with some added notes) is this:
00000000000264f0 <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0>:
264f0: 41 56 push %r14
264f2: 41 55 push %r13
264f4: 41 54 push %r12
264f6: 49 89 f4 mov %rsi,%r12 org-start
264f9: 55 push %rbp
264fa: 48 89 fd mov %rdi,%rbp here
264fd: 53 push %rbx
264fe: 48 83 ec 20 sub $0x20,%rsp
26502: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
26509: 00 00
2650b: 48 89 44 24 18 mov %rax,0x18(%rsp)
26510: 48 8b 05 d1 2a 27 00 mov 0x272ad1(%rip),%rax # 298fe8 <_DYNAMIC+0x1f8>
26517: 48 8b 18 mov (%rax),%rbx
2651a: ff 93 b8 14 00 00 callq *0x14b8(%rbx) goto-char
26520: 48 8d 74 24 08 lea 0x8(%rsp),%rsi
26525: bf 01 00 00 00 mov $0x1,%edi
2652a: 4c 8b 35 af 2a 27 00 mov 0x272aaf(%rip),%r14 # 298fe0 <_DYNAMIC+0x1f0>
26531: 49 8b 86 c8 00 00 00 mov 0xc8(%r14),%rax
26538: 48 89 44 24 08 mov %rax,0x8(%rsp)
2653d: ff 93 08 1a 00 00 callq *0x1a08(%rbx) c-beginning-of-macro
26543: 48 85 c0 test %rax,%rax
26546: 74 52 je 2659a <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0+0xaa>
26548: ff 93 68 14 00 00 callq *0x1468(%rbx) point
2654e: 49 89 c5 mov %rax,%r13 here-BOM
26551: 48 85 c0 test %rax,%rax
26554: 74 44 je 2659a <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0+0xaa>
26556: 4c 89 e7 mov %r12,%rdi org-start
26559: ff 93 b8 14 00 00 callq *0x14b8(%rbx) goto-char
2655f: bf 01 00 00 00 mov $0x1,%edi
26564: 48 8d 74 24 10 lea 0x10(%rsp),%rsi
26569: 49 8b 86 c8 00 00 00 mov 0xc8(%r14),%rax
26570: 48 89 44 24 10 mov %rax,0x10(%rsp)
26575: ff 93 08 1a 00 00 callq *0x1a08(%rbx) c-beginning-of-macro
2657b: 48 89 c7 mov %rax,%rdi
2657e: 48 85 c0 test %rax,%rax
26581: 74 09 je 2658c <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0+0x9c>
26583: ff 93 68 14 00 00 callq *0x1468(%rbx) point
26589: 48 89 c7 mov %rax,%rdi
2658c: 4c 89 ee mov %r13,%rsi here-BOM
2658f: ff 93 60 27 00 00 callq *0x2760(%rbx) eq
26595: 48 85 c0 test %rax,%rax <========================================================
26598: 74 03 je 2659d <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0+0xad>
2659a: 48 89 e8 mov %rbp,%rax here
2659d: 48 8b 54 24 18 mov 0x18(%rsp),%rdx
265a2: 64 48 2b 14 25 28 00 sub %fs:0x28,%rdx
265a9: 00 00
265ab: 75 0d jne 265ba <F632d64657465726d696e652d6c696d69742d6e6f2d6d6163726f_c_determine_limit_no_macro_0+0xca>
265ad: 48 83 c4 20 add $0x20,%rsp
265b1: 5b pop %rbx
265b2: 5d pop %rbp
265b3: 41 5c pop %r12
265b5: 41 5d pop %r13
265b7: 41 5e pop %r14
265b9: c3 retq
265ba: e8 41 12 fe ff callq 7800 <__stack_chk_fail <at> plt>
265bf: 90 nop
After the indicated line (0x26595), when 0x0 (nil) is in rax (i.e. the
`eq' function has returned nil) the result of the function should be
here-BOM, i.e. r13. There is no instruction
mov %r13,%rax
to effect this return. Instead, rax is still holding nil, and this is
falsely returned.
> --
> Alan Mackenzie (Nuremberg, Germany).
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