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[PATCH, c-family] Fix a PCH thinko (and thus PR61250).


When we are parsing a source file, the very first token might
be a PRAGMA_GCC_PCH_PREPROCESS.  This indicates that we are going
read in a PCH file (named as the value of the pragma).  If we don't
see this pragma, then we know that it's OK to release any resources
that the host might have set aside for the PCH file.

There is a thinko in the current implementation, in that the decision
to release resources is happening unconditionally right after the first
token is extracted but before it's been checked or acted upon.

This leads to the pch bug on Darwin, because we actually do release
resources - which are subsequently (reasonably) assumed to be available
when reading a PCH file.  We then get random crashes or hangs depending
on the interaction between unmmap and malloc.

The bug is present everywhere but doesn't show on (say) Linux, since
the release of PCH resources is a NOP there.

This effects all the c-family front ends, because they all use c_lex_with_flags ()
to implement this.

The solution is to check for the PRAGMA_GCC_PCH_PREPROCESS and only call
c_common_no_more_pch () when that is not the first token.

A secondary effect of the collection is that the name of the PCH file
can be collected during the ggc_pch_read() reset of state.  Therefore
we should issue any diagnostic that might name the file before the
collections are triggered.

Tested on x86-64-darwin16/18 (where the pch.exp tests fail roughly half the
time for any parallel testing) and pass reliably without it.
Tested on x86_64,powerpc-linux-gnu with no regressions (and no expectation
of any progression either).

Since the fix is in common code, it needs the ack of both C and C++ to apply
(I’m obviously OK with applying it from the Objective-C/C++ PoV)

OK for trunk?

given that this is a  show-stopper for PCH + -save-temps I would also like
to fix it on the open branches?



2019-08-22  Iain Sandoe  <>

	PR pch/61250
	* c-lex.c (c_lex_with_flags):  Don't call
	c_common_no_more_pch () from here.


2019-08-22  Iain Sandoe  <>

	PR pch/61250
	* c-parser.c (c_parse_file): Call c_common_no_more_pch ()
	after determining that the first token is not


2019-08-22  Iain Sandoe  <>

	PR pch/61250
	* parser.c (cp_parser_initial_pragma): Call c_common_no_more_pch ()
	after determining that the first token is not


2019-08-22  Iain Sandoe  <>

	PR pch/61250
	* ggc-page.c (ggc_pch_read): Read the ggc_pch_ondisk structure
	and issue any diagnostics needed before collecting the pre-PCH

 gcc/c-family/c-lex.c | 7 -------
 gcc/c/c-parser.c     | 2 ++
 gcc/cp/parser.c      | 5 ++++-
 gcc/ggc-page.c       | 5 +++--
 4 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/gcc/c-family/c-lex.c b/gcc/c-family/c-lex.c
index 851fd704e5..e3c602fbb8 100644
--- a/gcc/c-family/c-lex.c
+++ b/gcc/c-family/c-lex.c
@@ -394,7 +394,6 @@ enum cpp_ttype
 c_lex_with_flags (tree *value, location_t *loc, unsigned char *cpp_flags,
 		  int lex_flags)
-  static bool no_more_pch;
   const cpp_token *tok;
   enum cpp_ttype type;
   unsigned char add_flags = 0;
@@ -628,12 +627,6 @@ c_lex_with_flags (tree *value, location_t *loc, unsigned char *cpp_flags,
   if (cpp_flags)
     *cpp_flags = tok->flags | add_flags;
-  if (!no_more_pch)
-    {
-      no_more_pch = true;
-      c_common_no_more_pch ();
-    }
   timevar_pop (TV_CPP);
   return type;
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 81919a89cc..cf973b3c8d 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -20232,6 +20232,8 @@ c_parse_file (void)
   if (c_parser_peek_token (&tparser)->pragma_kind == PRAGMA_GCC_PCH_PREPROCESS)
     c_parser_pragma_pch_preprocess (&tparser);
+  else
+    c_common_no_more_pch ();
   the_parser = ggc_alloc<c_parser> ();
   *the_parser = tparser;
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index dbbfe1dbc2..ab8a0cabb4 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -41353,7 +41353,10 @@ cp_parser_initial_pragma (cp_token *first_token)
   cp_lexer_get_preprocessor_token (NULL, first_token);
   if (cp_parser_pragma_kind (first_token) != PRAGMA_GCC_PCH_PREPROCESS)
-    return;
+    {
+      c_common_no_more_pch ();
+      return;
+    }
   cp_lexer_get_preprocessor_token (NULL, first_token);
   if (first_token->type == CPP_STRING)
diff --git a/gcc/ggc-page.c b/gcc/ggc-page.c
index a2736bc1df..220f20c5cf 100644
--- a/gcc/ggc-page.c
+++ b/gcc/ggc-page.c
@@ -2556,6 +2556,9 @@ ggc_pch_read (FILE *f, void *addr)
   count_old_page_tables = G.by_depth_in_use;
+  if (fread (&d, sizeof (d), 1, f) != 1)
+    fatal_error (input_location, "cannot read PCH file: %m");
   /* We've just read in a PCH file.  So, every object that used to be
      allocated is now free.  */
   clear_marks ();
@@ -2584,8 +2587,6 @@ ggc_pch_read (FILE *f, void *addr)
   /* Allocate the appropriate page-table entries for the pages read from
      the PCH file.  */
-  if (fread (&d, sizeof (d), 1, f) != 1)
-    fatal_error (input_location, "cannot read PCH file: %m");
   for (i = 0; i < NUM_ORDERS; i++)

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