| // Copyright 2016 The Go Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| package ssa |
| |
| import ( |
| "fmt" |
| ) |
| |
| type loop struct { |
| header *Block // The header node of this (reducible) loop |
| outer *loop // loop containing this loop |
| |
| // By default, children exits, and depth are not initialized. |
| children []*loop // loops nested directly within this loop. Initialized by assembleChildren(). |
| exits []*Block // exits records blocks reached by exits from this loop. Initialized by findExits(). |
| |
| // Loops aren't that common, so rather than force regalloc to keep |
| // a map or slice for its data, just put it here. |
| spills []*Value |
| scratch int32 |
| |
| // Next three fields used by regalloc and/or |
| // aid in computation of inner-ness and list of blocks. |
| nBlocks int32 // Number of blocks in this loop but not within inner loops |
| depth int16 // Nesting depth of the loop; 1 is outermost. Initialized by calculateDepths(). |
| isInner bool // True if never discovered to contain a loop |
| |
| // register allocation uses this. |
| containsCall bool // if any block in this loop or any loop it contains has a call |
| } |
| |
| // outerinner records that outer contains inner |
| func (sdom SparseTree) outerinner(outer, inner *loop) { |
| oldouter := inner.outer |
| if oldouter == nil || sdom.isAncestorEq(oldouter.header, outer.header) { |
| inner.outer = outer |
| outer.isInner = false |
| if inner.containsCall { |
| outer.setContainsCall() |
| } |
| } |
| } |
| |
| func (l *loop) setContainsCall() { |
| for ; l != nil && !l.containsCall; l = l.outer { |
| l.containsCall = true |
| } |
| |
| } |
| func (l *loop) checkContainsCall(bb *Block) { |
| if bb.Kind == BlockDefer { |
| l.setContainsCall() |
| return |
| } |
| for _, v := range bb.Values { |
| if opcodeTable[v.Op].call { |
| l.setContainsCall() |
| return |
| } |
| } |
| } |
| |
| type loopnest struct { |
| f *Func |
| b2l []*loop |
| po []*Block |
| sdom SparseTree |
| loops []*loop |
| |
| // Record which of the lazily initialized fields have actually been initialized. |
| initializedChildren, initializedDepth, initializedExits bool |
| } |
| |
| func min8(a, b int8) int8 { |
| if a < b { |
| return a |
| } |
| return b |
| } |
| |
| func max8(a, b int8) int8 { |
| if a > b { |
| return a |
| } |
| return b |
| } |
| |
| const ( |
| blDEFAULT = 0 |
| blMin = blDEFAULT |
| blCALL = 1 |
| blRET = 2 |
| blEXIT = 3 |
| ) |
| |
| var bllikelies [4]string = [4]string{"default", "call", "ret", "exit"} |
| |
| func describePredictionAgrees(b *Block, prediction BranchPrediction) string { |
| s := "" |
| if prediction == b.Likely { |
| s = " (agrees with previous)" |
| } else if b.Likely != BranchUnknown { |
| s = " (disagrees with previous, ignored)" |
| } |
| return s |
| } |
| |
| func describeBranchPrediction(f *Func, b *Block, likely, not int8, prediction BranchPrediction) { |
| f.Config.Warnl(b.Line, "Branch prediction rule %s < %s%s", |
| bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction)) |
| } |
| |
| func likelyadjust(f *Func) { |
| // The values assigned to certain and local only matter |
| // in their rank order. 0 is default, more positive |
| // is less likely. It's possible to assign a negative |
| // unlikeliness (though not currently the case). |
| certain := make([]int8, f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit |
| local := make([]int8, f.NumBlocks()) // for our immediate predecessors. |
| |
| po := f.postorder() |
| nest := f.loopnest() |
| b2l := nest.b2l |
| |
| for _, b := range po { |
| switch b.Kind { |
| case BlockExit: |
| // Very unlikely. |
| local[b.ID] = blEXIT |
| certain[b.ID] = blEXIT |
| |
| // Ret, it depends. |
| case BlockRet, BlockRetJmp: |
| local[b.ID] = blRET |
| certain[b.ID] = blRET |
| |
| // Calls. TODO not all calls are equal, names give useful clues. |
| // Any name-based heuristics are only relative to other calls, |
| // and less influential than inferences from loop structure. |
| case BlockDefer: |
| local[b.ID] = blCALL |
| certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID]) |
| |
| default: |
| if len(b.Succs) == 1 { |
| certain[b.ID] = certain[b.Succs[0].b.ID] |
| } else if len(b.Succs) == 2 { |
| // If successor is an unvisited backedge, it's in loop and we don't care. |
| // Its default unlikely is also zero which is consistent with favoring loop edges. |
| // Notice that this can act like a "reset" on unlikeliness at loops; the |
| // default "everything returns" unlikeliness is erased by min with the |
| // backedge likeliness; however a loop with calls on every path will be |
| // tagged with call cost. Net effect is that loop entry is favored. |
| b0 := b.Succs[0].b.ID |
| b1 := b.Succs[1].b.ID |
| certain[b.ID] = min8(certain[b0], certain[b1]) |
| |
| l := b2l[b.ID] |
| l0 := b2l[b0] |
| l1 := b2l[b1] |
| |
| prediction := b.Likely |
| // Weak loop heuristic -- both source and at least one dest are in loops, |
| // and there is a difference in the destinations. |
| // TODO what is best arrangement for nested loops? |
| if l != nil && l0 != l1 { |
| noprediction := false |
| switch { |
| // prefer not to exit loops |
| case l1 == nil: |
| prediction = BranchLikely |
| case l0 == nil: |
| prediction = BranchUnlikely |
| |
| // prefer to stay in loop, not exit to outer. |
| case l == l0: |
| prediction = BranchLikely |
| case l == l1: |
| prediction = BranchUnlikely |
| default: |
| noprediction = true |
| } |
| if f.pass.debug > 0 && !noprediction { |
| f.Config.Warnl(b.Line, "Branch prediction rule stay in loop%s", |
| describePredictionAgrees(b, prediction)) |
| } |
| |
| } else { |
| // Lacking loop structure, fall back on heuristics. |
| if certain[b1] > certain[b0] { |
| prediction = BranchLikely |
| if f.pass.debug > 0 { |
| describeBranchPrediction(f, b, certain[b0], certain[b1], prediction) |
| } |
| } else if certain[b0] > certain[b1] { |
| prediction = BranchUnlikely |
| if f.pass.debug > 0 { |
| describeBranchPrediction(f, b, certain[b1], certain[b0], prediction) |
| } |
| } else if local[b1] > local[b0] { |
| prediction = BranchLikely |
| if f.pass.debug > 0 { |
| describeBranchPrediction(f, b, local[b0], local[b1], prediction) |
| } |
| } else if local[b0] > local[b1] { |
| prediction = BranchUnlikely |
| if f.pass.debug > 0 { |
| describeBranchPrediction(f, b, local[b1], local[b0], prediction) |
| } |
| } |
| } |
| if b.Likely != prediction { |
| if b.Likely == BranchUnknown { |
| b.Likely = prediction |
| } |
| } |
| } |
| // Look for calls in the block. If there is one, make this block unlikely. |
| for _, v := range b.Values { |
| if opcodeTable[v.Op].call { |
| local[b.ID] = blCALL |
| certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID]) |
| } |
| } |
| } |
| if f.pass.debug > 2 { |
| f.Config.Warnl(b.Line, "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin]) |
| } |
| |
| } |
| } |
| |
| func (l *loop) String() string { |
| return fmt.Sprintf("hdr:%s", l.header) |
| } |
| |
| func (l *loop) LongString() string { |
| i := "" |
| o := "" |
| if l.isInner { |
| i = ", INNER" |
| } |
| if l.outer != nil { |
| o = ", o=" + l.outer.header.String() |
| } |
| return fmt.Sprintf("hdr:%s%s%s", l.header, i, o) |
| } |
| |
| // nearestOuterLoop returns the outer loop of loop most nearly |
| // containing block b; the header must dominate b. loop itself |
| // is assumed to not be that loop. For acceptable performance, |
| // we're relying on loop nests to not be terribly deep. |
| func (l *loop) nearestOuterLoop(sdom SparseTree, b *Block) *loop { |
| var o *loop |
| for o = l.outer; o != nil && !sdom.isAncestorEq(o.header, b); o = o.outer { |
| } |
| return o |
| } |
| |
| func loopnestfor(f *Func) *loopnest { |
| po := f.postorder() |
| sdom := f.sdom() |
| b2l := make([]*loop, f.NumBlocks()) |
| loops := make([]*loop, 0) |
| |
| // Reducible-loop-nest-finding. |
| for _, b := range po { |
| if f.pass.debug > 3 { |
| fmt.Printf("loop finding (0) at %s\n", b) |
| } |
| |
| var innermost *loop // innermost header reachable from this block |
| |
| // IF any successor s of b is in a loop headed by h |
| // AND h dominates b |
| // THEN b is in the loop headed by h. |
| // |
| // Choose the first/innermost such h. |
| // |
| // IF s itself dominates b, the s is a loop header; |
| // and there may be more than one such s. |
| // Since there's at most 2 successors, the inner/outer ordering |
| // between them can be established with simple comparisons. |
| for _, e := range b.Succs { |
| bb := e.b |
| l := b2l[bb.ID] |
| |
| if sdom.isAncestorEq(bb, b) { // Found a loop header |
| if l == nil { |
| l = &loop{header: bb, isInner: true} |
| loops = append(loops, l) |
| b2l[bb.ID] = l |
| l.checkContainsCall(bb) |
| } |
| } else { // Perhaps a loop header is inherited. |
| // is there any loop containing our successor whose |
| // header dominates b? |
| if l != nil && !sdom.isAncestorEq(l.header, b) { |
| l = l.nearestOuterLoop(sdom, b) |
| } |
| } |
| |
| if l == nil || innermost == l { |
| continue |
| } |
| |
| if innermost == nil { |
| innermost = l |
| continue |
| } |
| |
| if sdom.isAncestor(innermost.header, l.header) { |
| sdom.outerinner(innermost, l) |
| innermost = l |
| } else if sdom.isAncestor(l.header, innermost.header) { |
| sdom.outerinner(l, innermost) |
| } |
| } |
| |
| if innermost != nil { |
| b2l[b.ID] = innermost |
| innermost.checkContainsCall(b) |
| innermost.nBlocks++ |
| } |
| } |
| |
| ln := &loopnest{f: f, b2l: b2l, po: po, sdom: sdom, loops: loops} |
| |
| // Curious about the loopiness? "-d=ssa/likelyadjust/stats" |
| if f.pass.stats > 0 && len(loops) > 0 { |
| ln.assembleChildren() |
| ln.calculateDepths() |
| ln.findExits() |
| |
| // Note stats for non-innermost loops are slightly flawed because |
| // they don't account for inner loop exits that span multiple levels. |
| |
| for _, l := range loops { |
| x := len(l.exits) |
| cf := 0 |
| if !l.containsCall { |
| cf = 1 |
| } |
| inner := 0 |
| if l.isInner { |
| inner++ |
| } |
| |
| f.LogStat("loopstats:", |
| l.depth, "depth", x, "exits", |
| inner, "is_inner", cf, "is_callfree", l.nBlocks, "n_blocks") |
| } |
| } |
| |
| if f.pass.debug > 1 && len(loops) > 0 { |
| fmt.Printf("Loops in %s:\n", f.Name) |
| for _, l := range loops { |
| fmt.Printf("%s, b=", l.LongString()) |
| for _, b := range f.Blocks { |
| if b2l[b.ID] == l { |
| fmt.Printf(" %s", b) |
| } |
| } |
| fmt.Print("\n") |
| } |
| fmt.Printf("Nonloop blocks in %s:", f.Name) |
| for _, b := range f.Blocks { |
| if b2l[b.ID] == nil { |
| fmt.Printf(" %s", b) |
| } |
| } |
| fmt.Print("\n") |
| } |
| return ln |
| } |
| |
| // assembleChildren initializes the children field of each |
| // loop in the nest. Loop A is a child of loop B if A is |
| // directly nested within B (based on the reducible-loops |
| // detection above) |
| func (ln *loopnest) assembleChildren() { |
| if ln.initializedChildren { |
| return |
| } |
| for _, l := range ln.loops { |
| if l.outer != nil { |
| l.outer.children = append(l.outer.children, l) |
| } |
| } |
| ln.initializedChildren = true |
| } |
| |
| // calculateDepths uses the children field of loops |
| // to determine the nesting depth (outer=1) of each |
| // loop. This is helpful for finding exit edges. |
| func (ln *loopnest) calculateDepths() { |
| if ln.initializedDepth { |
| return |
| } |
| ln.assembleChildren() |
| for _, l := range ln.loops { |
| if l.outer == nil { |
| l.setDepth(1) |
| } |
| } |
| ln.initializedDepth = true |
| } |
| |
| // findExits uses loop depth information to find the |
| // exits from a loop. |
| func (ln *loopnest) findExits() { |
| if ln.initializedExits { |
| return |
| } |
| ln.calculateDepths() |
| b2l := ln.b2l |
| for _, b := range ln.po { |
| l := b2l[b.ID] |
| if l != nil && len(b.Succs) == 2 { |
| sl := b2l[b.Succs[0].b.ID] |
| if recordIfExit(l, sl, b.Succs[0].b) { |
| continue |
| } |
| sl = b2l[b.Succs[1].b.ID] |
| if recordIfExit(l, sl, b.Succs[1].b) { |
| continue |
| } |
| } |
| } |
| ln.initializedExits = true |
| } |
| |
| // recordIfExit checks sl (the loop containing b) to see if it |
| // is outside of loop l, and if so, records b as an exit block |
| // from l and returns true. |
| func recordIfExit(l, sl *loop, b *Block) bool { |
| if sl != l { |
| if sl == nil || sl.depth <= l.depth { |
| l.exits = append(l.exits, b) |
| return true |
| } |
| // sl is not nil, and is deeper than l |
| // it's possible for this to be a goto into an irreducible loop made from gotos. |
| for sl.depth > l.depth { |
| sl = sl.outer |
| } |
| if sl != l { |
| l.exits = append(l.exits, b) |
| return true |
| } |
| } |
| return false |
| } |
| |
| func (l *loop) setDepth(d int16) { |
| l.depth = d |
| for _, c := range l.children { |
| c.setDepth(d + 1) |
| } |
| } |