[gcc r12-4532] Restore --param=max-fsm-thread-length

[gcc r12-4532] Restore --param=max-fsm-thread-length

[Aldy Hernandez]

https://gcc.gnu.org/g:82cd78f2c31db1664ca154d7fcd24e9eaee1427f

commit r12-4532-g82cd78f2c31db1664ca154d7fcd24e9eaee1427f
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Wed Oct 20 09:05:23 2021 +0200

    Restore --param=max-fsm-thread-length
    
    The removal of --param=max-fsm-thread-length is causing code
    explosion.  I thought that --param=max-fsm-thread-path-insns was a
    better gague for path profitability than raw BB length, but it turns
    out that we don't take into account PHIs when estimating the number of
    statements.
    
    In this PR, we have a sequence of very large PHIs that have us
    traversing extremely large paths that blow up the compilation.
    
    We could fix this a couple of different ways.  We could avoid
    traversing more than a certain number of PHI arguments, or ignore
    large PHIs altogether.  The old implementation certainly had this
    knob, and we could cut things off before we even got to the ranger.
    We could also adjust the instruction estimation to take into account
    PHIs, but I'm sure we'll mess something else in the process ;-).
    
    The easiest thing to do is just restore the knob.
    
    At a later time we could tweak this further, for instance,
    disregarding empty blocks in the count.  BTW, this is the reason I
    didn't chop things off in the lowlevel registry for all threaders: the
    forward threader can't really explore too deep paths, but it could
    theoretically get there while threading over empty blocks.
    
    This fixes 102814, 102852, and I bet it solves the Linux kernel cross
    compile issue.
    
    Tested on x86-64 Linux.
    
    gcc/ChangeLog:
    
            PR tree-optimization/102814
            * doc/invoke.texi: Document --param=max-fsm-thread-length.
            * params.opt: Add --param=max-fsm-thread-length.
            * tree-ssa-threadbackward.c
            (back_threader_profitability::profitable_path_p): Fail on paths
            longer than max-fsm-thread-length.

Diff:
---
 gcc/doc/invoke.texi           | 3 +++
 gcc/params.opt                | 4 ++++
 gcc/tree-ssa-threadbackward.c | 9 +++++++++
 3 files changed, 16 insertions(+)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0cc8a8edd05..c93d822431f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -14468,6 +14468,9 @@ Emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit().
 Maximum number of instructions to copy when duplicating blocks on a
 finite state automaton jump thread path.
 
+@item max-fsm-thread-length
+Maximum number of basic blocks on a jump thread path.
+
 @item parloops-chunk-size
 Chunk size of omp schedule for loops parallelized by parloops.
 
diff --git a/gcc/params.opt b/gcc/params.opt
index 06a6fdc9deb..83b3db6fea6 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -533,6 +533,10 @@ The maximum number of nested indirect inlining performed by early inliner.
 Common Joined UInteger Var(param_max_fields_for_field_sensitive) Param
 Maximum number of fields in a structure before pointer analysis treats the structure as a single variable.
 
+-param=max-fsm-thread-length=
+Common Joined UInteger Var(param_max_fsm_thread_length) Init(10) IntegerRange(1, 999999) Param Optimization
+Maximum number of basic blocks on a jump thread path.
+
 -param=max-fsm-thread-path-insns=
 Common Joined UInteger Var(param_max_fsm_thread_path_insns) Init(100) IntegerRange(1, 999999) Param Optimization
 Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 8770be88706..e378adbbf53 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -620,6 +620,15 @@ back_threader_profitability::profitable_path_p (const vec<basic_block> &m_path,
   if (m_path.length () <= 1)
       return false;
 
+  if (m_path.length () > (unsigned) param_max_fsm_thread_length)
+    {
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	fprintf (dump_file, "  FAIL: Jump-thread path not considered: "
+		 "the number of basic blocks on the path "
+		 "exceeds PARAM_MAX_FSM_THREAD_LENGTH.\n");
+      return false;
+    }
+
   int n_insns = 0;
   gimple_stmt_iterator gsi;
   loop_p loop = m_path[0]->loop_father;