Given a structure and 3 field access
typedef struct network
{
char inputfile[400];
int* nodes;
int* arcs;
int* stop_arcs;
} network_t;
int *arc;
int *node = net->nodes; <--- A
void *stop = (void *)net->stop_arcs; <--- B
for( arc = net->arcs; arc != (int *)stop; arc++ ) <--- C
GCC generates following instruction sequence in thumb mode with options -O2
-Os, it needs 9 insts to load 3 fields
mov r2, #200 <--- A1
lsl r1, r2, #1 <--- A2
.loc 1 14 0
mov r4, #204 <---- B1
lsl r3, r4, #1 <--- B2
.loc 1 13 0
ldr r2, [r0, r1] <---- A3
.loc 1 15 0
mov r1, #202 <--- C1
.loc 1 14 0
ldr r4, [r0, r3] <--- B3
.loc 1 15 0
lsl r3, r1, #1 <--- C2
ldr r3, [r0, r3] <--- C3
A better method is adjusting the base address first, which is nearer to all 3
fields we will access. Then we can use ldr dest, [base, offset] to load each
fields with only 1 instruction.
Although this opportunity is found in target ARM, it should also be applicable
to other architectures with addressing mode of (base + offset) and offset has a
limited value range.
--
Summary: inefficient address calculation of fields of large
struct
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carrot at google dot com
GCC build triplet: i686-linux
GCC host triplet: i686-linux
GCC target triplet: arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40314
