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| {{NAVCompiladores|{{TOCright}}}}
| | #REDIRECT [[ist:Optimization Topics]] |
| == Basic Blocks ==
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| Instruction sequence where control flow starts at the first instruction and ends at the last, without jumps (except at the last instruction).
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| Transformations in a basic block maintain the block semantics (considered as an atomic entity).
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| == Optimization Levels ==
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| * User - algorithm- and application-level optimizations
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| * Generic - considers intermediate code (processor-independent)
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| * Peephole - window over a series of consecutive instructions
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| * Local - optimization within a basic block
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| * Inter-block (information flow between blocks, mainly cycles)
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| * Global - multiple jumps (jumps to jumps, jumps to the next instruction)
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| = Machine-independent optimizations =
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| == Constant folding ==
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| Direct evaluation of expressions using only literal and known values:
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| int *x = (int *)malloc(1024 * 4);
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| int y = 3;
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| int z = 20 * y + 10;
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| Becomes:
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| int *x = (int *)malloc(4096);
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| int y = 3;
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| int z = 70;
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| == Elimination of Common Subexpressions ==
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| Using temporary variables to store common results:
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| d = c * (a + b)
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| e = (a + b) / 2
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| Becomes:
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| t = a + b
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| d = c * t
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| e = t / 2
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| == Cycles / Loops ==
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| === Loop unrolling ===
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| for (i = 1; i < n; i++)
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| a[i] = b[i] + 10;
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| Becomes:
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| for (i = 1; i < (n/4)*4; i+=4) {
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| a[i] = b[i] + 10;
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| a[i+1] = b[i+1] + 10;
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| a[i+2] = b[i+2] + 10;
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| a[i+3] = b[i+3] + 10;
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| }
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|
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| // rest of the cycle
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| for (; i < n; i++)
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| a[i] = b[i] + 10;
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| === Moving invariant code ===
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| for (i = 1; i < n; i++) {
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| a[i] = b[i] + c * d;
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| e = g[k];
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| }
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| Becomes:
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| t = c * d;
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| for (i = 1; i < n; i++) {
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| a[i] = b[i] + t;
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| }
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| e = g[k];
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| === Variable induction ===
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| for (i = 1; i < n; i++)
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| k = i * 4 + m;
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| Becomes:
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| k = m;
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| for (i = 1; i < n; i++)
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| k = k + 4;
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| === Other ===
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| = Machine-dependent Optimizations =
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| == Algebraic Simplification ==
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| xor eax,eax
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| instead of
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| mov 0,eax
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| == Strength Reduction ==
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| x << 2 + x
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| instead of
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| x * 5
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| == Instruction Reordering ==
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| Reorder independent instructions in order to avoid register spilling.
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| == Other ==
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| [[category:Compiladores]]
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| [[category:Ensino]]
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