I am by no means a GCC internals expert, but I'll give it a shot. Unfortunately most of the information on GCCs register allocation and spilling seems to be out of date (referencing files like local-alloc.c
that don't exist anymore).
I'm looking at the source code of gcc-4.5-20110825
.
In GNU C Compiler Internals it is mentioned that the initial function code is generated by expand_function_start
in gcc/function.c
. There we find the following for handling parameters:
4462 /* Initialize rtx for parameters and local variables.
4463 In some cases this requires emitting insns. */
4464 assign_parms (subr);
In assign_parms
the code that handles where each arguments is stored is the following:
3207 if (assign_parm_setup_block_p (&data))
3208 assign_parm_setup_block (&all, parm, &data);
3209 else if (data.passed_pointer || use_register_for_decl (parm))
3210 assign_parm_setup_reg (&all, parm, &data);
3211 else
3212 assign_parm_setup_stack (&all, parm, &data);
assign_parm_setup_block_p
handles aggregate data types and is not applicable in this case and since the data is not passed as a pointer GCC checks use_register_for_decl
.
Here the relevant part is:
1972 if (optimize)
1973 return true;
1974
1975 if (!DECL_REGISTER (decl))
1976 return false;
DECL_REGISTER
tests whether the variable was declared with the register
keyword. And now we have our answer: Most parameters live on the stack when optimizations are not enabled, and are then handled by assign_parm_setup_stack
. The route taken through the source code before it ends up spilling the value is slightly more complicated for pointer arguments, but can be traced in the same file if you're curious.
Why does GCC spill all arguments and local variables with optimizations disabled? To help debugging. Consider this simple function:
1 extern int bar(int);
2 int foo(int a) {
3 int b = bar(a | 1);
4 b += 42;
5 return b;
6 }
Compiled with gcc -O1 -c
this generates the following on my machine:
0: 48 83 ec 08 sub $0x8,%rsp
4: 83 cf 01 or $0x1,%edi
7: e8 00 00 00 00 callq c <foo+0xc>
c: 83 c0 2a add $0x2a,%eax
f: 48 83 c4 08 add $0x8,%rsp
13: c3 retq
Which is fine except if you break on line 5 and try to print the value of a, you get
(gdb) print a
$1 = <value optimized out>
As the argument gets overwritten since it's not used after the call to bar
.