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 <rguent...@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
