![Iterating vs non-Iterating
loop 1
<bb 3>:
# i_1 = PHI <i_4(2), i_7(3)>
# val_2 = PHI <val_5(2), val_6(3)>
val_6 = val_2 + 1;
i_7 = i_1 + 1;
if (i_7 < n_3)
goto <bb 3>; [INV]
else
goto <bb 4>; [INV]
<bb 4>:
_8 = val_6;
return _8;
ENTRY
<bb 2>:
n_3 = 1;
i_4 = 0;
val_5 = 0;](https://image.slidesharecdn.com/2022-cauldron-value-numbering-240215154708-605f14ba/85/2022-Cauldron-Value-Numbering-for-gcc-versions-11-320.jpg)
2022 Cauldron Value Numbering for gcc versions
- 1. Value Numbering in GCC Dr. Richard Biener SUSE Labs, Sep 15th, 2022
- 2. Value Numbering I Assign value numbers to expressions I Expressions that produce the same value should have the same value number I Usually achieved by hashing of simplified and canonicalized expressions with operands replaced by their value number
- 3. Value Numbering in GCC Multiple value numbering implementations and their main users I RTL CSE (cselib) I RTL PRE I GIMPLE SSA DOM (scoped tables) I GIMPLE SSA FRE/PRE (RPO VN) I simpler forms of VN in CCP and copy propagation
- 4. Common Subexpression Elimination For each statement I try to simplify the computed expression using value numbers of the operands I lookup value number of the simplified expression I if found and a register with that value is available, replace the expression with the register or constant I if not found, record a new value number for it and make it available in the destination receiving the value of the expression
- 5. Availability Different ways to track, update and query availability of a so called leader for a value number I with a DOM walk a value to leader map can be kept up-to-date with an unwind stack I the RPO VN walk records a list of leaders for each value that can be unwound when iterating and otherwise queried with dominator checks
- 6. Availability and expression simplification I use match.pd based simplification I value expression operands get substituted with their leaders I allows to keep flow-sensitive info like ranges
- 7. Memory Expressions ENTRY <bb 2>: # .MEM_3 = VDEF <.MEM_1(D)> p_2(D)->a = 0; # .MEM_4 = VDEF <.MEM_3> p_2(D)->b = 1; # .MEM_5 = VDEF <.MEM_4> x = *p_2(D); # VUSE <.MEM_5> _6 = x.a; # .MEM_7 = VDEF <.MEM_5> x ={v} {CLOBBER(eol)}; # VUSE <.MEM_7> return _6;
- 8. Memory Expressions I memory state is part of hashing, the current .MEM_n virtual definition is used I at lookup time walk the virtual SSA use->def chains, skip clobbers that do not alias and perform lookups with the previous memory state I fancy tricks during walking I memory to memory copies I pieces from larger entities I larger objects formed from smaller entities I memory handling consumes the majority of compile time
- 9. Why RPO VN I SSA SCC VN I reduces what to iterate I difficult to mate with CFG: not executable parts, predication, equivalences, region I RPO VN I iteration more costly I maps to the CFG, allows for flow-sensitive optimizations easily I allows region-based operation
- 10. RPO VN Operation Modes I can operate with different effort for memory handling I can do optimistic, iterating VN with elimination done after the fact I can do non-iterating VN with immediate elimination I can operate on the whole function or a single entry, multiple exit region
- 11. Iterating vs non-Iterating loop 1 <bb 3>: # i_1 = PHI <i_4(2), i_7(3)> # val_2 = PHI <val_5(2), val_6(3)> val_6 = val_2 + 1; i_7 = i_1 + 1; if (i_7 < n_3) goto <bb 3>; [INV] else goto <bb 4>; [INV] <bb 4>: _8 = val_6; return _8; ENTRY <bb 2>: n_3 = 1; i_4 = 0; val_5 = 0;
- 12. Iteration scheme I SSA SCC based VN iterates SSA SCCs until nothing changes I RPO VN iterates CFG cycles I rev_post_order_and_mark_dfs_back_seme can compute a RPO with CFG cycles adjacent and their extent in the RPO array recorded I handles irreducible regions, loop info would not I optimal regions for iteration I avoid iteration when possible, do not iterate until nothing changes I unwind cost to the iteration point linear with the amound of things to undo (expression hashes, availability) I iteration itself is O(n * loop-depth), inner cycles are iterated fully before iterating outer cycles
- 13. Non-iterative mode I Greedy walk along edges discovered as executable, but enforcing RPO visiting of reachable blocks. I Predecessors not visited and reachable from blocks later in RPO have to be conservatively assumed reachable. I Handles PHIs with unreachable incoming non-back edges optimally
- 14. RPO VN as Utility RPO VN was designed to be usable on small regions of a function without much overhead when doing that very often and with being much cheaper than a pass over the whole function. I loop unrolling applies CSE on unrolled bodies before trying to unroll the containing loop I loop if-conversion applies CSE to optimize predicates I unroll-and-jam applies CSE to leverage cross loop redundancies I uninit analysis uses RPO VN to compute basic block reachability without performing actual CSE
- 15. RPO VN Utility API enum vn_lookup_kind { VN_NOWALK, VN_WALK, VN_WALKREWRITE }; unsigned do_rpo_vn (function *fun, edge entry, bitmap exits, /* iterate */ bool = false, /* eliminate */ bool = true vn_lookup_kind = VN_WALKREWRITE); rev_post_order_and_mark_dfs_back_seme (function *fn, edge entry, bitmap exit_bbs, bool for_iteration, int *rev_post_order, vec<std::pair<int, int> > *scc_ext); auto_bb_flag, auto_edge_flag
- 16. RPO VN Utility Efficiency Non-iterating region-based VN with or without elimination was designed to be efficient I startup cost linear in the size of the region I performing RPO VN with VN_NOWALK, without iteration and elimination on each basic-block individually vs. performing a single RPO VN on the whole function is only around 15% slower for cc1files with insn-attrtab.i being the outlier at 280% I more elaborate memory handling or doing elimination does not allow for an apples vs. apples comparison I while doing CSE on the whole function might perform more optimizations doing that should never be faster than only doing CSE on the regions a pass performed a transformation on
- 17. TODO I experiment with using ranger instead of the ad-hoc predication we have I review equivalence tracking changes I think of a cheaper way to do “iteration” I we have simple DCE with a SSA worklist, need region DCE/DSE
- 18. Questions?