Confusion in gcc switch case assembly
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My Prog.c contains a simple switch case as follows:
switch (x)
{
case 1:
p=2;
break;
case 2:
p=3;
break;
case 3:
p=4;
break;
case 4:
p=5;
break;
case 5:
p=6;
break;
default:
break;
}
I compiled this program with
gcc -g -v prog.c
and created objdump with
objdump -S -l a.out>dump_file
For the switch case statement, I have found that one indirect branch has been created (jmp *%eax).
But if I remove any one of the case statement, no indirect branch is created.
gcc version: 5.4.0
I could not understand why this is happening?
c
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
add a comment |
My Prog.c contains a simple switch case as follows:
switch (x)
{
case 1:
p=2;
break;
case 2:
p=3;
break;
case 3:
p=4;
break;
case 4:
p=5;
break;
case 5:
p=6;
break;
default:
break;
}
I compiled this program with
gcc -g -v prog.c
and created objdump with
objdump -S -l a.out>dump_file
For the switch case statement, I have found that one indirect branch has been created (jmp *%eax).
But if I remove any one of the case statement, no indirect branch is created.
gcc version: 5.4.0
I could not understand why this is happening?
c
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
1
It might be useful to show us the output. Currently you could simply doif (x>=1 && x<=5) p=x+1;or jump to an address calculated fromx. This does not work if you have a gap in the cases list.
– Gerhardh
Nov 22 '18 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44
add a comment |
My Prog.c contains a simple switch case as follows:
switch (x)
{
case 1:
p=2;
break;
case 2:
p=3;
break;
case 3:
p=4;
break;
case 4:
p=5;
break;
case 5:
p=6;
break;
default:
break;
}
I compiled this program with
gcc -g -v prog.c
and created objdump with
objdump -S -l a.out>dump_file
For the switch case statement, I have found that one indirect branch has been created (jmp *%eax).
But if I remove any one of the case statement, no indirect branch is created.
gcc version: 5.4.0
I could not understand why this is happening?
c
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
My Prog.c contains a simple switch case as follows:
switch (x)
{
case 1:
p=2;
break;
case 2:
p=3;
break;
case 3:
p=4;
break;
case 4:
p=5;
break;
case 5:
p=6;
break;
default:
break;
}
I compiled this program with
gcc -g -v prog.c
and created objdump with
objdump -S -l a.out>dump_file
For the switch case statement, I have found that one indirect branch has been created (jmp *%eax).
But if I remove any one of the case statement, no indirect branch is created.
gcc version: 5.4.0
I could not understand why this is happening?
c
c
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
edited Nov 22 '18 at 13:45
EsmaeelE
1,17221222
1,17221222
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
asked Nov 22 '18 at 11:50
Moumita DasMoumita Das
91
91
1
It might be useful to show us the output. Currently you could simply doif (x>=1 && x<=5) p=x+1;or jump to an address calculated fromx. This does not work if you have a gap in the cases list.
– Gerhardh
Nov 22 '18 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44
add a comment |
1
It might be useful to show us the output. Currently you could simply doif (x>=1 && x<=5) p=x+1;or jump to an address calculated fromx. This does not work if you have a gap in the cases list.
– Gerhardh
Nov 22 '18 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44
1
1
It might be useful to show us the output. Currently you could simply do
if (x>=1 && x<=5) p=x+1; or jump to an address calculated from x. This does not work if you have a gap in the cases list.– Gerhardh
Nov 22 '18 at 12:39
It might be useful to show us the output. Currently you could simply do
if (x>=1 && x<=5) p=x+1; or jump to an address calculated from x. This does not work if you have a gap in the cases list.– Gerhardh
Nov 22 '18 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44
add a comment |
1 Answer
1
active
oldest
votes
This is happen because of Branch Tables. to avoid it compile with
gcc -g -v -fno-jump-tables SO.c
If you compile with jump tables [a feature in gcc compiler] size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.
Jump tables helps to reduce object code size by optimize and relocate some jump instructions.
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
This is happen because of Branch Tables. to avoid it compile with
gcc -g -v -fno-jump-tables SO.c
If you compile with jump tables [a feature in gcc compiler] size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.
Jump tables helps to reduce object code size by optimize and relocate some jump instructions.
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
add a comment |
This is happen because of Branch Tables. to avoid it compile with
gcc -g -v -fno-jump-tables SO.c
If you compile with jump tables [a feature in gcc compiler] size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.
Jump tables helps to reduce object code size by optimize and relocate some jump instructions.
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
add a comment |
This is happen because of Branch Tables. to avoid it compile with
gcc -g -v -fno-jump-tables SO.c
If you compile with jump tables [a feature in gcc compiler] size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.
Jump tables helps to reduce object code size by optimize and relocate some jump instructions.
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
This is happen because of Branch Tables. to avoid it compile with
gcc -g -v -fno-jump-tables SO.c
If you compile with jump tables [a feature in gcc compiler] size of dump_file is: 9,965 bytes
But without it size is: 11,493 bytes.
Jump tables helps to reduce object code size by optimize and relocate some jump instructions.
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
edited Nov 22 '18 at 12:58
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
answered Nov 22 '18 at 12:47
EsmaeelEEsmaeelE
1,17221222
1,17221222
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
add a comment |
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see stackoverflow.com/a/15799358/7508077
– EsmaeelE
Nov 22 '18 at 12:52
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
see avrfreaks.net/forum/using-indirect-jump-c
– EsmaeelE
Nov 22 '18 at 13:22
add a comment |
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1
It might be useful to show us the output. Currently you could simply do
if (x>=1 && x<=5) p=x+1;or jump to an address calculated fromx. This does not work if you have a gap in the cases list.– Gerhardh
Nov 22 '18 at 12:39
Compiler optimization decide to use indirect jumps than multiple comparisons.
– EsmaeelE
Nov 22 '18 at 12:40
maybe similar stackoverflow.com/a/11668346/7508077
– EsmaeelE
Nov 22 '18 at 12:44