Understanding the hardware and using it efficiently – Computing resources of a CPU – Pipelining – Branch prediction and hardware loop unrolling ● Conditional code vs efficiency ● Optimizing conditional v2[i+2]: v1[i+2]: a[i+2]:Branchless Computing 24 BRANCH PREDICTION: ANTIDOTE TO BRANCHES ● Well-pipelined code: a += v1[i] + v2[i] ● CPUs have branch predi ctorsBranchless Computing 25 LOOP UNROLLING cmp[i]:v3[i]==0 jump if true a[i]:a+=v1[i]+v2[i] jump a[i]:a+=v1[i]*v2[i] ...Branchless Computing 28 BRANCH PREDICTION: ANTIDOTE TO BRANCHES ● Speculatively pipelined code: a += (v3[i]) ? (v1[i]+v2[i]) :

bool cond) { int var = 0; // branch 1 if (p != nullptr) { var = 1; } // branch 2 if (cond) { var = 2; p = nullptr; } // branch 3 if (var == 1) { *p = 42; Not taking branch 1, but taking branch 3 • All warnings on infeasible paths are noise • Need info on the whole state! void path_sensitive(int *p, bool cond) { int var = 0; // branch 1 if (p (p != nullptr) { var = 1; } // branch 2 if (cond) { var = 2; p = nullptr; } // branch 3 if (var == 1) { *p = 42; // Null dereference? } }Flow-sensitive analysis

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, so julia> x = 3 3CHAPTER 9. CONTROL FLOW 92 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, so julia> x = 3 3CHAPTER 9. CONTROL FLOW 92 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,

many other languages. This value is simply the return value of the last executed statement in the branch that was chosen, soCHAPTER 10. CONTROL FLOW 97 julia> x = 3 3 julia> if x > 0 "positive!" else LTS version of Julia is versioned according to SemVer as v1.6.x; this branch will continue to receive bugfixes until a new LTS branch is chosen, at which point the v1.6.x series will no longer received arguments are evaluated first. In some cases, using ifelse instead of an if statement can eliminate the branch in generated code and provide higher performance in tight loops. Examples julia> ifelse(1 > 2,