https://gcc.gnu.org/g:8a8a022c81b6105f70f72dfe2dc7b368353b49e6
commit r17-3187-g8a8a022c81b6105f70f72dfe2dc7b368353b49e6 Author: Aldy Hernandez <[email protected]> Date: Mon Aug 10 06:59:36 2026 +0000 Add --param=dom-jump-threading to disable DOM jump threading [PR126103] As discussed in PR126103, the first step to removing DOM is untangling its threader from the rest of DOM. I've started auditing what we're missing in PRE + backwards threader, and need a way to disable DOM's threading for the barrage of PRs I'm about to file. We already have a way to disable all jump threads, but no way to disable just DOM's. This patch does jhust this with --param=dom-jump-threading=[01]. It's in line with what we temporarily did for ranger-threading and ranger VRP. The --param disables jump threading in DOM, but everything else DOM does is unaffected. Tested on ppc64le Linux. PR tree-optimization/126103 gcc/ChangeLog: * doc/params.texi (dom-jump-threading): Document. * params.opt (-param=dom-jump-threading=): New. * tree-ssa-dom.cc (dom_opt_dom_walker::after_dom_children): Honor param_dom_jump_threading. gcc/testsuite/ChangeLog: * gcc.dg/tree-ssa/dom-jump-threading-1.c: New test. * gcc.dg/tree-ssa/dom-jump-threading-2.c: New test. Diff: --- gcc/doc/params.texi | 6 ++++++ gcc/params.opt | 4 ++++ .../gcc.dg/tree-ssa/dom-jump-threading-1.c | 24 ++++++++++++++++++++++ .../gcc.dg/tree-ssa/dom-jump-threading-2.c | 22 ++++++++++++++++++++ gcc/tree-ssa-dom.cc | 3 ++- 5 files changed, 58 insertions(+), 1 deletion(-) diff --git a/gcc/doc/params.texi b/gcc/doc/params.texi index 26b34a59aa98..e69be2d2923e 100644 --- a/gcc/doc/params.texi +++ b/gcc/doc/params.texi @@ -894,6 +894,12 @@ optimizing. Maximum number of dominators BBs to walk when simplifying loop bounds and conditions. +@paindex dom-jump-threading +@item dom-jump-threading +Enable jump threading in the dominator optimizer pass. Disabling it +is useful to isolate the jump threads found by the backward threader, +which runs regardless. + @paindex max-jump-thread-duplication-stmts @item max-jump-thread-duplication-stmts Maximum number of statements allowed in a block that needs to be diff --git a/gcc/params.opt b/gcc/params.opt index 044c4a10bc44..0c6a3e514574 100644 --- a/gcc/params.opt +++ b/gcc/params.opt @@ -138,6 +138,10 @@ Probability that COMDAT function is shared with a different compilation unit. Common Joined UInteger Var(param_cxx_max_namespaces_for_diagnostic_help) Init(1000) Param Maximum number of namespaces to search for alternatives when name lookup fails. +-param=dom-jump-threading= +Common Joined UInteger Var(param_dom_jump_threading) Init(1) IntegerRange(0, 1) Param Optimization +Enable jump threading in the dominator optimizer pass. + -param=dse-max-alias-queries-per-store= Common Joined UInteger Var(param_dse_max_alias_queries_per_store) Init(256) Param Optimization Maximum number of queries into the alias oracle per store. diff --git a/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-1.c b/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-1.c new file mode 100644 index 000000000000..a0d0809b1763 --- /dev/null +++ b/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-1.c @@ -0,0 +1,24 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 --param=dom-jump-threading=0 -fdump-tree-dom2-details -fdump-tree-optimized" } */ + +/* Verify that --param=dom-jump-threading=0 keeps DOM from threading + jumps. Only DOM can thread a PHI of compares (the backward + threader cannot resolve the exit conditional to a single edge), so + with the param off the join block and its PHI must survive. */ + +void g (void); + +void +f (int x, int a, int b, int c, int d) +{ + _Bool t; + if (x) + t = a < b; + else + t = c < d; + if (t) + g (); +} + +/* { dg-final { scan-tree-dump-not "Registering jump thread" "dom2" } } */ +/* { dg-final { scan-tree-dump "PHI <" "optimized" } } */ diff --git a/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-2.c b/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-2.c new file mode 100644 index 000000000000..fb6b8a12732c --- /dev/null +++ b/gcc/testsuite/gcc.dg/tree-ssa/dom-jump-threading-2.c @@ -0,0 +1,22 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fdump-tree-dom2-details" } */ + +/* Same test as dom-jump-threading-1.c but with DOM jump threading at + its default (enabled): DOM must thread the PHI of compares, which + keeps the sibling test honest. */ + +void g (void); + +void +f (int x, int a, int b, int c, int d) +{ + _Bool t; + if (x) + t = a < b; + else + t = c < d; + if (t) + g (); +} + +/* { dg-final { scan-tree-dump "Registering jump thread" "dom2" } } */ diff --git a/gcc/tree-ssa-dom.cc b/gcc/tree-ssa-dom.cc index 02ffcbfc6771..7fded792a3c0 100644 --- a/gcc/tree-ssa-dom.cc +++ b/gcc/tree-ssa-dom.cc @@ -1801,7 +1801,8 @@ dom_opt_dom_walker::before_dom_children (basic_block bb) void dom_opt_dom_walker::after_dom_children (basic_block bb) { - m_threader->thread_outgoing_edges (bb); + if (param_dom_jump_threading) + m_threader->thread_outgoing_edges (bb); bitmap_clear_bit (m_state->get_blocks_on_stack (), bb->index); m_avail_exprs_stack->pop_to_marker (); m_const_and_copies->pop_to_marker ();
