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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org> To: gdb-prs@sourceware.org Subject: [Bug gdb/29996] New: [gdb/reverse] Reverse stepping skips first line in function in optimized code Date: Fri, 13 Jan 2023 11:25:20 +0000 [thread overview] Message-ID: <bug-29996-4717@http.sourceware.org/bugzilla/> (raw) https://sourceware.org/bugzilla/show_bug.cgi?id=29996 Bug ID: 29996 Summary: [gdb/reverse] Reverse stepping skips first line in function in optimized code Product: gdb Version: HEAD Status: NEW Severity: normal Priority: P2 Component: gdb Assignee: unassigned at sourceware dot org Reporter: vries at gcc dot gnu.org Target Milestone: --- Consider test-case test.c: ... $ cat -n test.c 1 int i, j, k; 2 int x, y, z; 3 4 void __attribute__((noinline, noclone)) 5 foo (void) 6 { 7 i = x; 8 j = y; 9 k = z; 10 } 11 12 int 13 main (void) 14 { 15 foo (); 16 return 0; 17 } ... Compiled with -O2 (using gcc 7.5.0): ... $ gcc -g -O2 test.c ... Let's step through the program, while recording: ... $ gdb -q a.out -ex start -ex record Reading symbols from a.out... Temporary breakpoint 1 at 0x4003c0: file test.c, line 15. Starting program: a.out Temporary breakpoint 1, main () at test.c:15 15 foo (); (gdb) step foo () at test.c:7 7 i = x; (gdb) step 8 j = y; (gdb) step 9 k = z; (gdb) step 10 } (gdb) ... So far, so good. Now let's try reverse stepping: ... 10 } (gdb) reverse-step 9 k = z; (gdb) reverse-step 8 j = y; (gdb) reverse-step No more reverse-execution history. main () at test.c:15 15 foo (); (gdb) ... Hmm, why did we skip over line 7? Minimal reproducer: ... $ gdb -q -batch a.out -ex start -ex record -ex "break 8" -ex continue \ -ex reverse-step Temporary breakpoint 1 at 0x4003c0: file test.c, line 15. Temporary breakpoint 1, main () at test.c:15 15 foo (); Breakpoint 2 at 0x4004bc: file test.c, line 8. Breakpoint 2, foo () at test.c:8 8 j = y; No more reverse-execution history. main () at test.c:15 15 foo (); ... It's because of this piece of code: ... /* When stepping backward, stop at beginning of line range (unless it's the function entry point, in which case keep going back to the call point). */ CORE_ADDR stop_pc = ecs->event_thread->stop_pc (); if (stop_pc == ecs->event_thread->control.step_range_start && stop_pc != ecs->stop_func_start && execution_direction == EXEC_REVERSE) end_stepping_range (ecs); else keep_going (ecs); ... which does the wrong thing for optimized code. For forward execution, the relevant logic is in skip_prologue_sal, see the locations_valid () test. -- You are receiving this mail because: You are on the CC list for the bug.
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