gdb - optimized value analysis - gdb

Question

My CPU is arm.How can I figure out the function parameter value if it's optimized out?
For example:
status_t NuPlayer::GenericSource::setDataSource(
int fd, int64_t offset, int64_t length) {
resetDataSource();
mFd = dup(fd);
mOffset = offset;
mLength = length;
Above function has 3 parameters, when I try to print the second parameter offset, I will get below result:
Thread 4 "Binder:15082_3" hit Breakpoint 1, android::NuPlayer::GenericSource::setDataSource (this=0xae63bb40, fd=8, offset=<optimized out>, length=9384436) at frameworks/av/media/libmediaplayerservice/nuplayer/GenericSource.cpp:123
123 resetDataSource();
(gdb) x/i $pc
=> 0xb02aaa80 <android::NuPlayer::GenericSource::setDataSource(int, long long, long long)+12>: blx 0xb0282454 <_ZN7android8NuPlayer13GenericSource15resetDataSourceEv#plt>
(gdb) n
125 mFd = dup(fd);
(gdb) print offset
$1 = <optimized out>
(gdb) p $eax
$2 = void
(gdb) disassemble /m
Dump of assembler code for function android::NuPlayer::GenericSource::setDataSource(int, long long, long long):
122 int fd, int64_t offset, int64_t length) {
0xb02aaa74 <+0>: push {r4, r5, r6, r7, lr}
0xb02aaa76 <+2>: sub sp, #4
0xb02aaa78 <+4>: mov r4, r3
0xb02aaa7a <+6>: mov r5, r2
0xb02aaa7c <+8>: mov r6, r1
0xb02aaa7e <+10>: mov r7, r0
123 resetDataSource();
=> 0xb02aaa80 <+12>: blx 0xb0282454 <_ZN7android8NuPlayer13GenericSource15resetDataSourceEv#plt>
124
125 mFd = dup(fd);
0xb02aaa84 <+16>: mov r0, r6
0xb02aaa86 <+18>: blx 0xb027e5d8 <dup#plt>
0xb02aaa8a <+22>: ldrd r2, r1, [sp, #24]
0xb02aaa8e <+26>: str.w r0, [r7, #224] ; 0xe0
0xb02aaa92 <+30>: movs r0, #0
126 mOffset = offset;
0xb02aaa94 <+32>: strd r5, r4, [r7, #232] ; 0xe8
127 mLength = length;
0xb02aaa98 <+36>: strd r2, r1, [r7, #240] ; 0xf0
128
129 // delay data source creation to prepareAsync() to avoid blocking
130 // the calling thread in setDataSource for any significant time.
131 return OK;
0xb02aaa9c <+40>: add sp, #4
0xb02aaa9e <+42>: pop {r4, r5, r6, r7, pc}
End of assembler dump.
(gdb)
I guess it's in some register but the result of $eax is void.

Answer 1

I guess it's in some register but the result of $eax is void.
There is no register called eax on ARM.
To know which register the parameter is in, you need to know calling convention.
Looks like you are using 32-bit ARM. From above link:
r0 to r3: used to hold argument values passed to a subroutine
So you should do info registers, verify that r0 == 0xae63bb40, r1 == 8 and find the offset in r2.

Answer 2

Sounds like example code has assigned the parameter variable to local variable already, so print that value will be exactly the same as optimized out parameters.
mOffset = offset;
mLength = length;