I am using a MIPS CPU (PIC32) in an embedded project, but I am starting to question my choice. I understand that a RISC CPU like MIPS will generate more instructions than one might expect, but I didn't think it would be like this. Here is a snippet from the disassembly listing:
225: LATDSET = 0x0040;
sw s1,24808(s2)
sw s4,24808(s2)
sw s4,24808(s2)
sw s1,24808(s2)
sw s4,24808(s3)
sw s4,24808(s3)
sw s1,24808(s3)
226: {
227: porte = PORTE;
lw t1,24848(s4)
andi v0,t1,0xffff
lw v1,24848(s6)
andi ra,v1,0xffff
lw v1,24848(s6)
andi ra,v1,0xffff
lw v0,24848(s6)
andi t2,v0,0xffff
lw a2,24848(s5)
andi v1,a2,0xffff
lw t2,24848(s5)
andi v1,t2,0xffff
lw v0,24848(s5)
andi t2,v0,0xffff
228: if (porte & 0x0004)
andi t2,v0,0x4
andi s8,ra,0x4
andi s8,ra,0x4
andi ra,t2,0x4
andi a1,v1,0x4
andi a2,v1,0x4
andi a2,t2,0x4
229: pst_bytes_somi[0] |= sliding_bit;
or t3,t4,s0
xori a3,t2,0x0
movz t3,s0,a3
addu s0,t3,zero
or t3,t4,s1
xori a3,s8,0x0
movz t3,s1,a3
addu s1,t3,zero
or t3,t4,s1
xori a3,s8,0x0
movz t3,s1,a3
addu s1,t3,zero
or v1,t4,s0
xori a3,ra,0x0
movz v1,s0,a3
addu s0,v1,zero
or a0,t4,s2
xori a3,a1,0x0
movz a0,s2,a3
addu s2,a0,zero
or t3,t4,s2
xori a3,a2,0x0
movz t3,s2,a3
addu s2,t3,zero
or v1,t4,s0
xori a3,a2,0x0
movz v1,s0,a3
This seems like a crazy number of instructions for simple reading / writing and testing variables at fixed addresses. On a different CPU, I could probably get each C statement down to about 1..3 instructions, without resorting to hand-written asm. Obviously the clock rate is fairly high, but it's not 10x higher than what I would have in a different CPU (e.g. dsPIC).
I have optimisation set to maximum. Is my C compiler terrible (It's gcc 3.4.4)? Or is this typical of MIPS?
Finally figured out the answer. The disassembly listing is totally misleading. The compiler is doing loop unrolling, and what we're seeing under each C statement is actually 8x the number of instructions, because it's unrolling the loop 8x. The instructions are not at consecutive addresses! Turning off loop unrolling in the compiler options produces this:
225: LATDSET = 0x0040;
sw s3,24808(s2)
226: {
227: porte = PORTE;
lw t1,24848(s5)
andi v0,t1,0xffff
228: if (porte & 0x0004)
andi t2,v0,0x4
229: pst_bytes_somi[0] |= sliding_bit;
or t3,t4,s0
xori a3,t2,0x0
movz t3,s0,a3
addu s0,t3,zero
230:
Panic over everyone.