What must the executable preserve?
Suppose x and y start at 5.0 and 3.0. The program assigns x = (x * 1.5) - y and prints x. The multiplication and subtraction have no separately observable results. If x is never used again, a compiler may replace the computation with output of the fixed value 4.5. If x is used later, its value must remain available. TAKEAWAY: Optimized execution need not follow every written step; it must preserve observable states. section 5.1
C describes execution through an abstract state machine. Its observable states include relevant stored values and output as they change. A compiler can rearrange or remove work only when the executable preserves those states. Type matters: combining two additions can be valid for an unsigned value yet invalid when an intermediate operation could raise an exception and terminate execution. section 5.1 section 5.1.4
x += 5; /* Do something else without x in the meantime. */ x += 7;
Inspect the two additions with unrelated work between them. Their apparent equivalence to one addition depends on whether an intermediate value is observed and on the type of x. section 5.1.4
Programmer-directed optimization needs equal care. Complicated changes made before there is evidence of a performance problem can damage readability, soundness, and maintainability. Speed is not a reason to accept out-of-bounds array access, uninitialized or expired objects, or integer overflow: the consequences can include crashes, incorrect results, and exposed information. TAKEAWAY: Do not trade safety for performance. section 15.0
Measure before judging a change, but treat timings as evidence with limitations. A timing call consumes time and can alter register handling, scheduling, caches, and compiler optimization opportunities. Repeat experiments and examine the mean and variation rather than trusting one run. If a loop is meant to measure work, first check that the compiler has not removed that work. section 15.3
Listing 15.1. Measuring several code snippets repeatedly
53 timespec_get(&t[0], TIME_UTC);
54 /* Volatile for i ensures that the loop is effected */
55 for (uint64_t volatile i = 0; i < iterations; ++i) {
56 /* do nothing */
57 }
58 timespec_get(&t[1], TIME_UTC);
59 /* s must be volatile to ensure that the loop is effected */
60 for (uint64_t i = 0; i < iterations; ++i) {
61 s = i;
62 }
63 timespec_get(&t[2], TIME_UTC);
64 /* Opaque computation ensures that the loop is effected */
65 for (uint64_t i = 1; accu0 < upper; i += 2) {
66 accu0 += i;
67 }
68 timespec_get(&t[3], TIME_UTC);
69 /* A function call can usually not be optimized out. */
70 for (uint64_t i = 0; i < iterations; ++i) {
71 timespec_get(&tdummy, TIME_UTC);
72 accu1 += tdummy.tv_nsec;
73 }
74 timespec_get(&t[4], TIME_UTC);
75 /* A function call can usually not be optimized out, but
76 an inline function can. */
77 for (uint64_t i = 0; i < iterations; ++i) {
78 timespec_get(&tdummy, TIME_UTC);
79 stats_collect1(&sdummy[1], tdummy.tv_nsec);
80 }
81 timespec_get(&t[5], TIME_UTC);
82 for (uint64_t i = 0; i < iterations; ++i) {
83 timespec_get(&tdummy, TIME_UTC);
84 stats_collect2(&sdummy[2], tdummy.tv_nsec);
85 }
86 timespec_get(&t[6], TIME_UTC);
87 for (uint64_t i = 0; i < iterations; ++i) {
88 timespec_get(&tdummy, TIME_UTC);
89 stats_collect3(&sdummy[3], tdummy.tv_nsec);
90 }
91 timespec_get(&t[7], TIME_UTC);
!@%STYLE%@!
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!@%STYLE%@!
Compare timed loops whose work is retained through volatile access, an opaque computation, or a function call. The listing makes the question of what is actually being timed concrete. section 15.3
