(New page: == Basic Blocks == Instruction sequence where control flow starts at the first instruction and ends at the last, without jumps (except at the last instruction). Transformations in a basi...) |
|||
Line 1: | Line 1: | ||
+ | {{TOCright}} | ||
== Basic Blocks == | == Basic Blocks == | ||
Instruction sequence where control flow starts at the first instruction and ends at the last, without jumps (except at the last instruction).
Transformations in a basic block maintain the block semantics (considered as an atomic entity).
Direct evaluation of expressions using only literal and known values:
int *x = (int *)malloc(1024 * 4); int y = 3; int z = 20 * y + 10;
Becomes:
int *x = (int *)malloc(4096); int y = 3; int z = 70;
Using temporary variables to store common results:
d = c * (a + b) e = (a + b) / 2
Becomes:
t = a + b d = c * t e = t / 2
for (i = 1; i < n; i++) a[i] = b[i] + 10;
Becomes:
for (i = 1; i < (n/4)*n; i+=4) { a[i] = b[i] + 10; a[i+1] = b[i+1] + 10; a[i+2] = b[i+2] + 10; a[i+3] = b[i+3] + 10; } // rest of the cycle for (; i < n; i++) a[i] = b[i] + 10;
for (i = 1; i < n; i++) { a[i] = b[i] + c * d; e = g[k]; }
Becomes:
t = c * d; for (i = 1; i < n; i++) { a[i] = b[i] + t; } e = g[k];
for (i = 1; i < n; i++) k = i * 4 + m;
Becomes:
k = m; for (i = 1; i < n; i++) k = k + 4;
xor eax,eax
instead of
mov 0,eax
x << 2 + x
instead of
x * 5
Reorder independent instructions in order to avoid register spilling.