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[dev.ssa] cmd/compile/internal/ssa: Update TODO list
Change-Id: Ibcd4c6984c8728fd9ab76e0c7df555984deaf281 Reviewed-on: https://go-review.googlesource.com/13471 Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
This commit is contained in:
Keith Randall
parent
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9787ba43ee
@ -1,71 +1,70 @@
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This is a list of things that need to be worked on. It is by no means complete.
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This is a list of things that need to be worked on. It will hopefully
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be complete soon.
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Allocation
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- Allocation of decls in stackalloc. Decls survive if they are
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addrtaken or are too large for registerization.
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Coverage
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--------
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- Floating point numbers
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- Complex numbers
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- Integer division
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- Fat objects (strings/slices/interfaces) vs. Phi
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- Defer?
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- Closure args
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- PHEAP vars
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Scheduling
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- Make sure loads are scheduled correctly with respect to stores.
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Same for flag type values. We can't have more than one value of
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mem or flag types live at once.
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- Reduce register pressure. Schedule instructions which kill
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variables first.
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Correctness
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-----------
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- GC maps
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- Write barriers
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- Debugging info
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- Handle flags register correctly (clobber/spill/restore)
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- Proper panic edges from checks & calls (+deferreturn)
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- Can/should we move control values out of their basic block?
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- Anything to do for the race detector?
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- Slicing details (avoid ptr to next object)
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Values
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- Store *Type instead of Type? Keep an array of used Types in Func
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and reference by id? Unify with the type ../gc so we just use a
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pointer instead of an interface?
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- Recycle dead values instead of using GC to do that.
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- A lot of Aux fields are just int64. Add a separate AuxInt field?
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If not that, then cache the interfaces that wrap int64s.
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- OpStore uses 3 args. Increase the size of argstorage to 3?
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Optimizations (better compiled code)
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------------------------------------
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- Reduce register pressure in scheduler
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- More strength reduction: multiply -> shift/add combos (Worth doing?)
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- Strength reduction: constant divides -> multiply
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- Expand current optimizations to all bit widths
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- Nil/bounds check removal
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- Combining nil checks with subsequent load
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- Implement memory zeroing with REPSTOSQ and DuffZero
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- Implement memory copying with REPMOVSQ and DuffCopy
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- Make deadstore work with zeroing
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- Branch prediction: Respect hints from the frontend, add our own
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- Add a value range propagation pass (for bounds elim & bitwidth reduction)
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- Stackalloc: group pointer-containing variables & spill slots together
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- Stackalloc: organize values to allow good packing
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- Regalloc: use arg slots as the home for arguments (don't copy args to locals)
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- Reuse stack slots for noninterfering & compatible values (but see issue 8740)
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- (x86) Combine loads into other ops
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- (x86) More combining address arithmetic into loads/stores
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Optimizations (better compiler)
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-------------------------------
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- Smaller Value.Type (int32 or ptr)? Get rid of types altogether?
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- Recycle dead Values (and Blocks) explicitly instead of using GC
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- OpStore uses 3 args. Increase the size of Value.argstorage to 3?
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- Constant cache
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- Reuseable slices (e.g. []int of size NumValues()) cached in Func
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Regalloc
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- Make less arch-dependent
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- Don't spill everything at every basic block boundary.
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- Allow args and return values to be ssa-able.
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- Handle 2-address instructions.
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- Floating point registers
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- Make calls clobber all registers
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- Make liveness analysis non-quadratic.
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- Handle in-place instructions (like XORQconst) directly:
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Use XORQ AX, 1 rather than MOVQ AX, BX; XORQ BX, 1.
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--------
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- Make less arch-dependent
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- Don't spill everything at every basic block boundary
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- Allow args and return values to be ssa-able
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- Handle 2-address instructions
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- Make calls clobber all registers
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- Make liveness analysis non-quadratic
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- Materialization of constants
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StackAlloc:
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- Sort variables so all ptr-containing ones are first (so stack
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maps are smaller)
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- Reuse stack slots for noninterfering and type-compatible variables
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(both AUTOs and spilled Values). But see issue 8740 for what
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"type-compatible variables" mean and what DWARF information provides.
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Rewrites
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- Strength reduction (both arch-indep and arch-dependent?)
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- Start another architecture (arm?)
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- 64-bit ops on 32-bit machines
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- <regwidth ops. For example, x+y on int32s on amd64 needs (MOVLQSX (ADDL x y)).
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Then add rewrites like (MOVLstore (MOVLQSX x) m) -> (MOVLstore x m)
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to get rid of most of the MOVLQSX.
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- Determine which nil checks can be done implicitly (by faulting)
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and which need code generated, and do the code generation.
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Common-Subexpression Elimination
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- Make better decision about which value in an equivalence class we should
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choose to replace other values in that class.
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- Can we move control values out of their basic block?
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This would break nilcheckelim as currently implemented,
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but it could be replaced by a similar CFG simplication pass.
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- Investigate type equality. During SSA generation, should we use n.Type or (say) TypeBool?
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Should we get rid of named types in favor of underlying types during SSA generation?
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Should we introduce a new type equality routine that is less strict than the frontend's?
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Other
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- Write barriers
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- For testing, do something more sophisticated than
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checkOpcodeCounts. Michael Matloob suggests using a similar
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pattern matcher to the rewrite engine to check for certain
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expression subtrees in the output.
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- Implement memory zeroing with REPSTOSQ and DuffZero
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- make deadstore work with zeroing.
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- Add a value range propagation optimization pass.
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Use it for bounds check elimination and bitwidth reduction.
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- Branch prediction: Respect hints from the frontend, add our own.
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Future/other
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------------
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- Start another architecture (arm?)
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- 64-bit ops on 32-bit machines
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- Investigate type equality. During SSA generation, should we use n.Type or (say) TypeBool?
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- Should we get rid of named types in favor of underlying types during SSA generation?
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- Should we introduce a new type equality routine that is less strict than the frontend's?
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- Infrastructure for enabling/disabling/configuring passes
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@ -30,6 +30,8 @@ func schedule(f *Func) {
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for _, b := range f.Blocks {
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// Find store chain for block.
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// Store chains for different blocks overwrite each other, so
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// the calculated store chain is good only for this block.
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for _, v := range b.Values {
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if v.Op != OpPhi && v.Type.IsMemory() {
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for _, w := range v.Args {
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