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[unavailable regs/locals, 0/11] Introduction
- From: Pedro Alves <pedro at codesourcery dot com>
- To: gdb-patches at sourceware dot org
- Date: Tue, 22 Feb 2011 13:27:22 +0000
- Subject: [unavailable regs/locals, 0/11] Introduction
This series mainly teaches GDB about unavailable/uncollected
args/locals properly. Currently, GDB knows about, and handles
gracefuly unavailable globals, but since the registers unavailable-ness
hasn't been glued yet with values and frames, any local whose
location is dependent on an unavailable register will get
either miscomputed, printed as garbage, or throw an error
instead of printing a graceful "<unavailable>".
E.g., I've placed a tracepoint on line 57, and collected
nothing:
56 void *thread_function0(void *arg) {
57 int my_number = (long) arg;
58 volatile int *myp = (volatile int *) &args[my_number];
59
60 /* Don't run forever. Run just short of it :) */
61 while (*myp > 0)
62 {
63 (*myp) ++;
64 usleep (1); /* Loop increment. */
(gdb) trace thread_function0
Tracepoint 2 at 0x400752: file threads.c, line 57.
(gdb) tstart
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
(gdb) tstop
(gdb) tfind
Found trace frame 0, tracepoint 2
#0 thread_function0 (arg=<unavailable>) at threads.c:57
57 register int my_number = (long) arg;
(gdb) info registers
rax 0x0 0
rbx 0x0 0
rcx 0x0 0
rdx 0x0 0
rsi 0x0 0
rdi 0x0 0
rbp 0x0 0x0
rsp 0x0 0x0
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0x400752 0x400752 <thread_function0+13>
eflags 0x0 [ ]
cs 0x0 0
ss 0x0 0
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
(gdb)
Inspecting the traceframe, one can see above
that gdbserver infers the $PC from the tracepoint address,
but all other registers are shown as 0, which is wrong.
We really don't know their value.
If one prints the locals in this case, we'll see
bogus values:
(gdb) info locals
my_number = 0
myp = <unavailable>
We don't really know my_number's value, so '0'
is wrong and misleading. Notice:
(gdb) info scope thread_function0
Scope for thread_function0:
Symbol arg is a variable at frame base reg $rsp offset 8+-56, length 8.
Symbol my_number is a variable in $rbx, length 4.
Symbol myp is a variable at frame base reg $rsp offset 8+-40, length 8.
So not even myp is correct:
(gdb) p $rsp
$2 = (void *) 0x0
(gdb) p &arg
$3 = (void **) 0xffffffffffffffe8
that's 0 + -40. Obviously not the correct address of
that local.
The series fixes this by glueing register unavailable-ness
with value unavailable-ness, and then value unavailable-ness
with frame unavailable-ness. Midway through the series, since
GDB will be able to distinguish a register value zero from
an unavailable register, GDB will start throwing errors
whenever such register values are used in computations.
The second part of the series makes GDB handle that
gracefuly when necessary (particularly, when the PC itself
is unavailable).
Tested on x86_64-linux, native and gdbserver. No
regressions, and the new tests pass cleanly.
--
Pedro Alves