OK .. So we think we know how to run a program.

[ajaygargnsit]

Hey, I just ported one of my programs from gcc to turbo c compiler, and noted a strange thing. I used a structure

struct s
{
int a;
char b;
}obj;

on gcc, sizeof(obj) gave 8, (4 for int, 1+3 for char due to padding);
while on turbo c, sizeof(obj) gave plain 3 (2 for int, 1 for char).

Now I took the value of a to fit into two bytes, and the program seemed to show same results with both compilers.

Now my question is : 1) Is the same machine code generated for both compilers.
2) If not, which is a faster version (as far as number of machine cycles are concerned).

[JosAH]

1) Is the same machine code generated for both compilers.

Not likely; one compiler decided an int to be two bytes, another wants an int
to be four bytes.

2) If not, which is a faster version (as far as number of machine cycles are concerned).

Who can tell? It depends on the processor and on the instructions generated
by the compilers.

kind regards,

Jos

[ajaygargnsit]

Hmm Ok, I think I need some more explanations. Ok Let's go step by step

1) Let's say we are using the gcc compiler, and a Pentium 4. Also say there is another machine running same gcc compiler, but a different processor, say Sunsparc. So, now, even though only the processor differs, yet different machine codes will be generated for the same code. (I assume it to be the case, kindly correct me I am wrong..)

Now how does the compiler know that what machine code to generate. (I presume it has got to do something with compiler "Adjusting" its parameters, during boot up time, BUT I AM NOT SURE... PLZ help..)

Looking forward to a reply.

Ajay Garg

[JosAH]

Hmm Ok, I think I need some more explanations. Ok Let's go step by step

1) Let's say we are using the gcc compiler, and a Pentium 4. Also say there is another machine running same gcc compiler, but a different processor, say Sunsparc. So, now, even though only the processor differs, yet different machine codes will be generated for the same code. (I assume it to be the case, kindly correct me I am wrong..)

Now how does the compiler know that what machine code to generate. (I presume it has got to do something with compiler "Adjusting" its parameters, during boot up time, BUT I AM NOT SURE... PLZ help..)

Looking forward to a reply.

Ajay Garg

The compiler itself is a (compiled) binary. You can't run a gcc for a Sparc on
a pentium, nor vice versa. See your other thread where I also explained things
a bit more. The compiler doesn't 'know' anything about other processors. It's
the code generating part of the compiler that knows all about one single processor.

kind regards,

Jos