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| 1 | +package ssa |
| 2 | + |
| 3 | +type indVar struct { |
| 4 | + ind *Value // induction variable |
| 5 | + inc *Value // increment, a constant |
| 6 | + nxt *Value // ind+inc variable |
| 7 | + min *Value // minimum value. inclusive, |
| 8 | + max *Value // maximum value. exclusive. |
| 9 | + entry *Block // entry block in the loop. |
| 10 | + // Invariants: for all blocks dominated by entry: |
| 11 | + // min <= ind < max |
| 12 | + // min <= nxt <= max |
| 13 | +} |
| 14 | + |
| 15 | +// findIndVar finds induction variables in a function. |
| 16 | +// |
| 17 | +// Look for variables and blocks that satisfy the following |
| 18 | +// |
| 19 | +// loop: |
| 20 | +// ind = (Phi min nxt), |
| 21 | +// if ind < max |
| 22 | +// then goto enter_loop |
| 23 | +// else goto exit_loop |
| 24 | +// |
| 25 | +// enter_loop: |
| 26 | +// do something |
| 27 | +// nxt = inc + ind |
| 28 | +// goto loop |
| 29 | +// |
| 30 | +// exit_loop: |
| 31 | +// |
| 32 | +// |
| 33 | +// TODO: handle 32 bit operations |
| 34 | +func findIndVar(f *Func, sdom sparseTree) []indVar { |
| 35 | + var iv []indVar |
| 36 | + |
| 37 | +nextb: |
| 38 | + for _, b := range f.Blocks { |
| 39 | + if b.Kind != BlockIf || len(b.Preds) != 2 { |
| 40 | + continue |
| 41 | + } |
| 42 | + |
| 43 | + var ind, max *Value // induction, and maximum |
| 44 | + entry := -1 // which successor of b enters the loop |
| 45 | + |
| 46 | + // Check thet the control if it either ind < max or max > ind. |
| 47 | + // TODO: Handle Leq64, Geq64. |
| 48 | + switch b.Control.Op { |
| 49 | + case OpLess64: |
| 50 | + entry = 0 |
| 51 | + ind, max = b.Control.Args[0], b.Control.Args[1] |
| 52 | + case OpGreater64: |
| 53 | + entry = 0 |
| 54 | + ind, max = b.Control.Args[1], b.Control.Args[0] |
| 55 | + default: |
| 56 | + continue nextb |
| 57 | + } |
| 58 | + |
| 59 | + // Check that the induction variable is a phi that depends on itself. |
| 60 | + if ind.Op != OpPhi { |
| 61 | + continue |
| 62 | + } |
| 63 | + |
| 64 | + // Extract min and nxt knowing that nxt is an addition (e.g. Add64). |
| 65 | + var min, nxt *Value // minimum, and next value |
| 66 | + if n := ind.Args[0]; n.Op == OpAdd64 && (n.Args[0] == ind || n.Args[1] == ind) { |
| 67 | + min, nxt = ind.Args[1], n |
| 68 | + } else if n := ind.Args[1]; n.Op == OpAdd64 && (n.Args[0] == ind || n.Args[1] == ind) { |
| 69 | + min, nxt = ind.Args[0], n |
| 70 | + } else { |
| 71 | + // Not a recognized induction variable. |
| 72 | + continue |
| 73 | + } |
| 74 | + |
| 75 | + var inc *Value |
| 76 | + if nxt.Args[0] == ind { // nxt = ind + inc |
| 77 | + inc = nxt.Args[1] |
| 78 | + } else if nxt.Args[1] == ind { // nxt = inc + ind |
| 79 | + inc = nxt.Args[0] |
| 80 | + } else { |
| 81 | + panic("unreachable") // one of the cases must be true from the above. |
| 82 | + } |
| 83 | + |
| 84 | + // Expect the increment to be a positive constant. |
| 85 | + // TODO: handle negative increment. |
| 86 | + if inc.Op != OpConst64 || inc.AuxInt <= 0 { |
| 87 | + continue |
| 88 | + } |
| 89 | + |
| 90 | + // Up to now we extracted the induction variable (ind), |
| 91 | + // the increment delta (inc), the temporary sum (nxt), |
| 92 | + // the mininum value (min) and the maximum value (max). |
| 93 | + // |
| 94 | + // We also know that ind has the form (Phi min nxt) where |
| 95 | + // nxt is (Add inc nxt) which means: 1) inc dominates nxt |
| 96 | + // and 2) there is a loop starting at inc and containing nxt. |
| 97 | + // |
| 98 | + // We need to prove that the induction variable is incremented |
| 99 | + // only when it's smaller than the maximum value. |
| 100 | + // Two conditions must happen listed below to accept ind |
| 101 | + // as an induction variable. |
| 102 | + |
| 103 | + // First condition: loop entry has a single predecessor, which |
| 104 | + // is the header block. This implies that b.Succs[entry] is |
| 105 | + // reached iff ind < max. |
| 106 | + if len(b.Succs[entry].Preds) != 1 { |
| 107 | + // b.Succs[1-entry] must exit the loop. |
| 108 | + continue |
| 109 | + } |
| 110 | + |
| 111 | + // Second condition: b.Succs[entry] dominates nxt so that |
| 112 | + // nxt is computed when inc < max, meaning nxt <= max. |
| 113 | + if !sdom.isAncestorEq(b.Succs[entry], nxt.Block) { |
| 114 | + // inc+ind can only be reached through the branch that enters the loop. |
| 115 | + continue |
| 116 | + } |
| 117 | + |
| 118 | + // If max is c + SliceLen with c <= 0 then we drop c. |
| 119 | + // Makes sure c + SliceLen doesn't overflow when SliceLen == 0. |
| 120 | + // TODO: save c as an offset from max. |
| 121 | + if w, c := dropAdd64(max); (w.Op == OpStringLen || w.Op == OpSliceLen) && 0 >= c && -c >= 0 { |
| 122 | + max = w |
| 123 | + } |
| 124 | + |
| 125 | + // We can only guarantee that the loops runs withing limits of induction variable |
| 126 | + // if the increment is 1 or when the limits are constants. |
| 127 | + if inc.AuxInt != 1 { |
| 128 | + ok := false |
| 129 | + if min.Op == OpConst64 && max.Op == OpConst64 { |
| 130 | + if max.AuxInt > min.AuxInt && max.AuxInt%inc.AuxInt == min.AuxInt%inc.AuxInt { // handle overflow |
| 131 | + ok = true |
| 132 | + } |
| 133 | + } |
| 134 | + if !ok { |
| 135 | + continue |
| 136 | + } |
| 137 | + } |
| 138 | + |
| 139 | + if f.pass.debug > 1 { |
| 140 | + if min.Op == OpConst64 { |
| 141 | + b.Func.Config.Warnl(b.Line, "Induction variable with minimum %d and increment %d", min.AuxInt, inc.AuxInt) |
| 142 | + } else { |
| 143 | + b.Func.Config.Warnl(b.Line, "Induction variable with non-const minimum and increment %d", inc.AuxInt) |
| 144 | + } |
| 145 | + } |
| 146 | + |
| 147 | + iv = append(iv, indVar{ |
| 148 | + ind: ind, |
| 149 | + inc: inc, |
| 150 | + nxt: nxt, |
| 151 | + min: min, |
| 152 | + max: max, |
| 153 | + entry: b.Succs[entry], |
| 154 | + }) |
| 155 | + b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max) |
| 156 | + } |
| 157 | + |
| 158 | + return iv |
| 159 | +} |
| 160 | + |
| 161 | +// loopbce performs loop based bounds check elimination. |
| 162 | +func loopbce(f *Func) { |
| 163 | + idom := dominators(f) |
| 164 | + sdom := newSparseTree(f, idom) |
| 165 | + ivList := findIndVar(f, sdom) |
| 166 | + |
| 167 | + m := make(map[*Value]indVar) |
| 168 | + for _, iv := range ivList { |
| 169 | + m[iv.ind] = iv |
| 170 | + } |
| 171 | + |
| 172 | + removeBoundsChecks(f, sdom, m) |
| 173 | +} |
| 174 | + |
| 175 | +// removesBoundsChecks remove IsInBounds and IsSliceInBounds based on the induction variables. |
| 176 | +func removeBoundsChecks(f *Func, sdom sparseTree, m map[*Value]indVar) { |
| 177 | + for _, b := range f.Blocks { |
| 178 | + if b.Kind != BlockIf { |
| 179 | + continue |
| 180 | + } |
| 181 | + |
| 182 | + v := b.Control |
| 183 | + |
| 184 | + // Simplify: |
| 185 | + // (IsInBounds ind max) where 0 <= const == min <= ind < max. |
| 186 | + // (IsSliceInBounds ind max) where 0 <= const == min <= ind < max. |
| 187 | + // Found in: |
| 188 | + // for i := range a { |
| 189 | + // use a[i] |
| 190 | + // use a[i:] |
| 191 | + // use a[:i] |
| 192 | + // } |
| 193 | + if v.Op == OpIsInBounds || v.Op == OpIsSliceInBounds { |
| 194 | + ind, add := dropAdd64(v.Args[0]) |
| 195 | + if ind.Op != OpPhi { |
| 196 | + goto skip1 |
| 197 | + } |
| 198 | + if v.Op == OpIsInBounds && add != 0 { |
| 199 | + goto skip1 |
| 200 | + } |
| 201 | + if v.Op == OpIsSliceInBounds && (0 > add || add > 1) { |
| 202 | + goto skip1 |
| 203 | + } |
| 204 | + |
| 205 | + if iv, has := m[ind]; has && sdom.isAncestorEq(iv.entry, b) && isNonNegative(iv.min) { |
| 206 | + if v.Args[1] == iv.max { |
| 207 | + if f.pass.debug > 0 { |
| 208 | + f.Config.Warnl(b.Line, "Found redundant %s", v.Op) |
| 209 | + } |
| 210 | + goto simplify |
| 211 | + } |
| 212 | + } |
| 213 | + } |
| 214 | + skip1: |
| 215 | + |
| 216 | + // Simplify: |
| 217 | + // (IsSliceInBounds ind (SliceCap a)) where 0 <= min <= ind < max == (SliceLen a) |
| 218 | + // Found in: |
| 219 | + // for i := range a { |
| 220 | + // use a[:i] |
| 221 | + // use a[:i+1] |
| 222 | + // } |
| 223 | + if v.Op == OpIsSliceInBounds { |
| 224 | + ind, add := dropAdd64(v.Args[0]) |
| 225 | + if ind.Op != OpPhi { |
| 226 | + goto skip2 |
| 227 | + } |
| 228 | + if 0 > add || add > 1 { |
| 229 | + goto skip2 |
| 230 | + } |
| 231 | + |
| 232 | + if iv, has := m[ind]; has && sdom.isAncestorEq(iv.entry, b) && isNonNegative(iv.min) { |
| 233 | + if v.Args[1].Op == OpSliceCap && iv.max.Op == OpSliceLen && v.Args[1].Args[0] == iv.max.Args[0] { |
| 234 | + if f.pass.debug > 0 { |
| 235 | + f.Config.Warnl(b.Line, "Found redundant %s (len promoted to cap)", v.Op) |
| 236 | + } |
| 237 | + goto simplify |
| 238 | + } |
| 239 | + } |
| 240 | + } |
| 241 | + skip2: |
| 242 | + |
| 243 | + continue |
| 244 | + |
| 245 | + simplify: |
| 246 | + f.Logf("removing bounds check %v at %v in %s\n", b.Control, b, f.Name) |
| 247 | + b.Kind = BlockFirst |
| 248 | + b.SetControl(nil) |
| 249 | + } |
| 250 | +} |
| 251 | + |
| 252 | +func dropAdd64(v *Value) (*Value, int64) { |
| 253 | + if v.Op == OpAdd64 && v.Args[0].Op == OpConst64 { |
| 254 | + return v.Args[1], v.Args[0].AuxInt |
| 255 | + } |
| 256 | + if v.Op == OpAdd64 && v.Args[1].Op == OpConst64 { |
| 257 | + return v.Args[0], v.Args[1].AuxInt |
| 258 | + } |
| 259 | + return v, 0 |
| 260 | +} |
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