Hi all,
at work we put embedded lm32 soft-core CPUs in FPGAs and write
the firmware in C.
At home I enjoy writing small projects in D from time to time,
but I don't consider myself a D expert.
Now, I'm trying to run some toy examples in D on the lm32 cpu.
I'm using a recent gcc-elf-lm32. I succeeded in compiling and
running some code and it works fine.
But I noticed, when calling a function with a string argument,
the string is not stored in registers, but on the stack.
Consider a simple function (below) that writes bytes to a
peripheral (that forwards the data to the host computer via USB).
I've two versions, an ideomatic D one, and another version where
pointer and length are two distinct function parameters.
I also show the generated assembly code. The string version is 4
instructions longer, just because of the stack manipulation. In
addition, it is also slower because it need to access the ram,
and it needs more stack space.
My question: Is there a way I can tell the D compiler to use
registers instead of stack for string arguments, or any other
trick to reduce code size while maintaining an ideomatic D
codestyle?
Best regards
Michael
// ideomatic D version
void write_to_host(in string msg) {
// a fixed address to get bytes to the host via usb
char *usb_slave = cast(char*)BaseAdr.ft232_slave;
foreach(ch; msg) {
*usb_slave = ch;
}
}
// resulting assembly code (compiled with -Os) 12 instructions
_D10firmware_d13write_to_hostFxAyaZv:
addi sp, sp, -8
addi r3, r0, 4096
sw (sp+4), r1
sw (sp+8), r2
add r1, r2, r1
.L3:
be r2,r1,.L1
lbu r4, (r2+0)
addi r2, r2, 1
sb (r3+0), r4
bi .L3
.L1:
addi sp, sp, 8
b ra
// C-like version
void write_to_hostC(const char *msg, int len) {
char *ptr = cast(char*)msg;
char *usb_slave = cast(char*)BaseAdr.ft232_slave;
while (len--) {
*usb_slave = *ptr++;
}
}
// resulting assembly code (compiled with -Os) 8 instructions
_D10firmware_d14write_to_hostCFxPaiZv:
add r2, r1, r2
addi r3, r0, 4096
.L7:
be r1,r2,.L5
lbu r4, (r1+0)
addi r1, r1, 1
sb (r3+0), r4
bi .L7
.L5:
b ra
