cmd/compile: improve debug locations for partially live in-params

During DWARF debug location generation, as a preamble to the main data
flow analysis, examine the function entry block to look for in-params
arriving in registers that are partially or completely dead, and
insert new OpArg{Int,Float}Reg values for the dead or partially-dead
pieces. In addition, add entries to the f.NamedValues table for
incoming live register-resident params that don't already have
entries. This helps create better/saner DWARF location expressions for
params. Example:

  func foo(s string, used int, notused int) int {
      return len(s) + used
  }

When optimization is complete for this function, the parameter
"notused" is completely dead, meaning that there is no entry for it in
the f.NamedValues table (which then means we don't emit a DWARF
variable location expression for it in the function enty block). In
addition, since only the length field of "s" is used, there is no
DWARF location expression for the other component of "s", leading to
degraded DWARF.

There are still problems/issues with DWARF location generation, but
this does improve things with respect to being able to print the
values of incoming parameters when stopped in the debugger at the
entry point of a function (when optimization is enabled).

Updates #40724.

Change-Id: I5bb5253648942f9fd33b081fe1a5a36208e75785
Reviewed-on: https://go-review.googlesource.com/c/go/+/322631
Trust: Than McIntosh <[email protected]>
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: thanm

src/cmd/compile/internal/ssa/debug.go

@@ -7,10 +7,13 @@ package ssa
 import (
 	"cmd/compile/internal/abi"
 	"cmd/compile/internal/ir"
+	"cmd/compile/internal/types"
 	"cmd/internal/dwarf"
 	"cmd/internal/obj"
+	"cmd/internal/src"
 	"encoding/hex"
 	"fmt"
+	"internal/buildcfg"
 	"math/bits"
 	"sort"
 	"strings"
@@ -335,6 +338,216 @@ func (s *debugState) stateString(state stateAtPC) string {
 	return strings.Join(strs, "")
 }
 
+// slotCanonicalizer is a table used to lookup and canonicalize
+// LocalSlot's in a type insensitive way (e.g. taking into account the
+// base name, offset, and width of the slot, but ignoring the slot
+// type).
+type slotCanonicalizer struct {
+	slmap  map[slotKey]SlKeyIdx
+	slkeys []LocalSlot
+}
+
+func newSlotCanonicalizer() *slotCanonicalizer {
+	return &slotCanonicalizer{
+		slmap:  make(map[slotKey]SlKeyIdx),
+		slkeys: []LocalSlot{LocalSlot{N: nil}},
+	}
+}
+
+type SlKeyIdx uint32
+
+const noSlot = SlKeyIdx(0)
+
+// slotKey is a type-insensitive encapsulation of a LocalSlot; it
+// is used to key a map within slotCanonicalizer.
+type slotKey struct {
+	name        *ir.Name
+	offset      int64
+	width       int64
+	splitOf     SlKeyIdx // idx in slkeys slice in slotCanonicalizer
+	splitOffset int64
+}
+
+// lookup looks up a LocalSlot in the slot canonicalizer "sc", returning
+// a canonical index for the slot, and adding it to the table if need
+// be. Return value is the canonical slot index, and a boolean indicating
+// whether the slot was found in the table already (TRUE => found).
+func (sc *slotCanonicalizer) lookup(ls LocalSlot) (SlKeyIdx, bool) {
+	split := noSlot
+	if ls.SplitOf != nil {
+		split, _ = sc.lookup(*ls.SplitOf)
+	}
+	k := slotKey{
+		name: ls.N, offset: ls.Off, width: ls.Type.Width,
+		splitOf: split, splitOffset: ls.SplitOffset,
+	}
+	if idx, ok := sc.slmap[k]; ok {
+		return idx, true
+	}
+	rv := SlKeyIdx(len(sc.slkeys))
+	sc.slkeys = append(sc.slkeys, ls)
+	sc.slmap[k] = rv
+	return rv, false
+}
+
+func (sc *slotCanonicalizer) canonSlot(idx SlKeyIdx) LocalSlot {
+	return sc.slkeys[idx]
+}
+
+// PopulateABIInRegArgOps examines the entry block of the function
+// and looks for incoming parameters that have missing or partial
+// OpArg{Int,Float}Reg values, inserting additional values in
+// cases where they are missing. Example:
+//
+//      func foo(s string, used int, notused int) int {
+//        return len(s) + used
+//      }
+//
+// In the function above, the incoming parameter "used" is fully live,
+// "notused" is not live, and "s" is partially live (only the length
+// field of the string is used). At the point where debug value
+// analysis runs, we might expect to see an entry block with:
+//
+//   b1:
+//     v4 = ArgIntReg <uintptr> {s+8} [0] : BX
+//     v5 = ArgIntReg <int> {used} [0] : CX
+//
+// While this is an accurate picture of the live incoming params,
+// we also want to have debug locations for non-live params (or
+// their non-live pieces), e.g. something like
+//
+//   b1:
+//     v9 = ArgIntReg <*uint8> {s+0} [0] : AX
+//     v4 = ArgIntReg <uintptr> {s+8} [0] : BX
+//     v5 = ArgIntReg <int> {used} [0] : CX
+//     v10 = ArgIntReg <int> {unused} [0] : DI
+//
+// This function examines the live OpArg{Int,Float}Reg values and
+// synthesizes new (dead) values for the non-live params or the
+// non-live pieces of partially live params.
+//
+func PopulateABIInRegArgOps(f *Func) {
+	pri := f.ABISelf.ABIAnalyzeFuncType(f.Type.FuncType())
+
+	// When manufacturing new slots that correspond to splits of
+	// composite parameters, we want to avoid creating a new sub-slot
+	// that differs from some existing sub-slot only by type, since
+	// the debug location analysis will treat that slot as a separate
+	// entity. To achieve this, create a lookup table of existing
+	// slots that is type-insenstitive.
+	sc := newSlotCanonicalizer()
+	for _, sl := range f.Names {
+		sc.lookup(*sl)
+	}
+
+	// Add slot -> value entry to f.NamedValues if not already present.
+	addToNV := func(v *Value, sl LocalSlot) {
+		values, ok := f.NamedValues[sl]
+		if !ok {
+			// Haven't seen this slot yet.
+			sla := f.localSlotAddr(sl)
+			f.Names = append(f.Names, sla)
+		} else {
+			for _, ev := range values {
+				if v == ev {
+					return
+				}
+			}
+		}
+		values = append(values, v)
+		f.NamedValues[sl] = values
+	}
+
+	newValues := []*Value{}
+
+	abiRegIndexToRegister := func(reg abi.RegIndex) int8 {
+		i := f.ABISelf.FloatIndexFor(reg)
+		if i >= 0 { // float PR
+			return f.Config.floatParamRegs[i]
+		} else {
+			return f.Config.intParamRegs[reg]
+		}
+	}
+
+	// Helper to construct a new OpArg{Float,Int}Reg op value.
+	var pos src.XPos
+	if len(f.Entry.Values) != 0 {
+		pos = f.Entry.Values[0].Pos
+	}
+	synthesizeOpIntFloatArg := func(n *ir.Name, t *types.Type, reg abi.RegIndex, sl LocalSlot) *Value {
+		aux := &AuxNameOffset{n, sl.Off}
+		op, auxInt := ArgOpAndRegisterFor(reg, f.ABISelf)
+		v := f.newValueNoBlock(op, t, pos)
+		v.AuxInt = auxInt
+		v.Aux = aux
+		v.Args = nil
+		v.Block = f.Entry
+		newValues = append(newValues, v)
+		addToNV(v, sl)
+		f.setHome(v, &f.Config.registers[abiRegIndexToRegister(reg)])
+		return v
+	}
+
+	// Make a pass through the entry block looking for
+	// OpArg{Int,Float}Reg ops. Record the slots they use in a table
+	// ("sc"). We use a type-insensitive lookup for the slot table,
+	// since the type we get from the ABI analyzer won't always match
+	// what the compiler uses when creating OpArg{Int,Float}Reg ops.
+	for _, v := range f.Entry.Values {
+		if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
+			aux := v.Aux.(*AuxNameOffset)
+			sl := LocalSlot{N: aux.Name, Type: v.Type, Off: aux.Offset}
+			// install slot in lookup table
+			idx, _ := sc.lookup(sl)
+			// add to f.NamedValues if not already present
+			addToNV(v, sc.canonSlot(idx))
+		} else if v.Op.IsCall() {
+			// if we hit a call, we've gone too far.
+			break
+		}
+	}
+
+	// Now make a pass through the ABI in-params, looking for params
+	// or pieces of params that we didn't encounter in the loop above.
+	for _, inp := range pri.InParams() {
+		if !isNamedRegParam(inp) {
+			continue
+		}
+		n := inp.Name.(*ir.Name)
+
+		// Param is spread across one or more registers. Walk through
+		// each piece to see whether we've seen an arg reg op for it.
+		types, offsets := inp.RegisterTypesAndOffsets()
+		for k, t := range types {
+			// Note: this recipe for creating a LocalSlot is designed
+			// to be compatible with the one used in expand_calls.go
+			// as opposed to decompose.go. The expand calls code just
+			// takes the base name and creates an offset into it,
+			// without using the SplitOf/SplitOffset fields. The code
+			// in decompose.go does the opposite -- it creates a
+			// LocalSlot object with "Off" set to zero, but with
+			// SplitOf pointing to a parent slot, and SplitOffset
+			// holding the offset into the parent object.
+			pieceSlot := LocalSlot{N: n, Type: t, Off: offsets[k]}
+
+			// Look up this piece to see if we've seen a reg op
+			// for it. If not, create one.
+			_, found := sc.lookup(pieceSlot)
+			if !found {
+				// This slot doesn't appear in the map, meaning it
+				// corresponds to an in-param that is not live, or
+				// a portion of an in-param that is not live/used.
+				// Add a new dummy OpArg{Int,Float}Reg for it.
+				synthesizeOpIntFloatArg(n, t, inp.Registers[k],
+					pieceSlot)
+			}
+		}
+	}
+
+	// Insert the new values into the head of the block.
+	f.Entry.Values = append(newValues, f.Entry.Values...)
+}
+
 // BuildFuncDebug returns debug information for f.
 // f must be fully processed, so that each Value is where it will be when
 // machine code is emitted.
@@ -349,6 +562,10 @@ func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset fu
 	state.stackOffset = stackOffset
 	state.ctxt = ctxt
 
+	if buildcfg.Experiment.RegabiArgs {
+		PopulateABIInRegArgOps(f)
+	}
+
 	if state.loggingEnabled {
 		state.logf("Generating location lists for function %q\n", f.Name)
 	}