[PATCH][RFC] c/94392 - only enable -ffinite-loops for C++

[Richard Biener <rguenther@suse.de>]

This does away with enabling -ffinite-loops at -O2+ for all languages
and instead enables it selectively for C++ only.

Jason, I didn't find a reference as to when the forward progress
guarantee was introduced to C++ so I randomly chose C++11, is that
correct?

Joseph, this simply never enables -ffinite-loops for C by default
also based on the fact this was added in the first place for C++
standard library abstraction removal thus without any real-world
C testcases.

Bootstrap / regtest in progress - I expect I need to add explicit
-ffinite-loops to a few testcases (and fixup the changelog).

I did not look whether other languages guarantee forward progress
but the feature is new in GCC 10 and restricting it to C++ is
sensible I think.

Any further comments?

Writing a runtime testcase is hard since by definition it would
not finish.  I'll construct a scan-tree-dump one.

Thanks,
Richard.

2020-04-01  Richard Biener  <rguenther@suse.de>

	PR c/94392
	* c-opts.c (): Enable -ffinite-loops for -O2 and C++11.

	* common.opt (ffinite-loops): Initialize to zero.
	* opts.c (default_options_table): Remove OPT_ffinite_loops
	entry.
	* ipa-inline.c (): Match up -ffinite-loops.
	* doc/invoke.texi (ffinite-loops): Adjust documentation of
	default setting.
---
 gcc/c-family/c-opts.c | 4 ++++
 gcc/common.opt        | 2 +-
 gcc/doc/invoke.texi   | 3 ++-
 gcc/ipa-inline.c      | 5 ++++-
 gcc/opts.c            | 1 -
 5 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 6b6c754ad86..58ba0948e79 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -989,6 +989,10 @@ c_common_post_options (const char **pfilename)
   SET_OPTION_IF_UNSET (&global_options, &global_options_set, flag_new_ttp,
 		       cxx_dialect >= cxx17);
 
+  /* C++11 guarantees forward progress.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set, flag_finite_loops,
+		       optimize >= 2 && cxx_dialect >= cxx11);
+
   if (cxx_dialect >= cxx11)
     {
       /* If we're allowing C++0x constructs, don't warn about C++98
diff --git a/gcc/common.opt b/gcc/common.opt
index 4368910cb54..bb2ea4c905d 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1490,7 +1490,7 @@ Common Report Var(flag_finite_math_only) Optimization SetByCombined
 Assume no NaNs or infinities are generated.
 
 ffinite-loops
-Common Report Var(flag_finite_loops) Optimization
+Common Report Var(flag_finite_loops) Optimization Init(0)
 Assume that loops with an exit will terminate and not loop indefinitely.
 
 ffixed-
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 412750c1fc9..142bfeacead 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -10432,7 +10432,8 @@ Assume that a loop with an exit will eventually take the exit and not loop
 indefinitely.  This allows the compiler to remove loops that otherwise have
 no side-effects, not considering eventual endless looping as such.
 
-This option is enabled by default at @option{-O2}.
+This option is enabled by default at @option{-O2} for C++ with -std=c++11
+or higher.
 
 @item -ftree-dominator-opts
 @opindex ftree-dominator-opts
diff --git a/gcc/ipa-inline.c b/gcc/ipa-inline.c
index 302ce16a646..d0e59d431ca 100644
--- a/gcc/ipa-inline.c
+++ b/gcc/ipa-inline.c
@@ -512,7 +512,10 @@ can_inline_edge_by_limits_p (struct cgraph_edge *e, bool report,
 	      /* When devirtualization is disabled for callee, it is not safe
 		 to inline it as we possibly mangled the type info.
 		 Allow early inlining of always inlines.  */
-	      || (!early && check_maybe_down (flag_devirtualize)))
+	      || (!early && check_maybe_down (flag_devirtualize))
+	      /* It's not safe to inline a function where loops maybe
+		 infinite into a function where we assume the reverse.  */
+	      || check_maybe_down (flag_finite_loops))
 	{
 	  e->inline_failed = CIF_OPTIMIZATION_MISMATCH;
 	  inlinable = false;
diff --git a/gcc/opts.c b/gcc/opts.c
index 5dc7d65dedd..d4df8627bf7 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -478,7 +478,6 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_2_PLUS, OPT_fdevirtualize, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fdevirtualize_speculatively, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fexpensive_optimizations, NULL, 1 },
-    { OPT_LEVELS_2_PLUS, OPT_ffinite_loops, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fgcse, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fhoist_adjacent_loads, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_findirect_inlining, NULL, 1 },
-- 
2.25.1