c: Better fix for speed up compilation of large char array initializers when not using #embed [PR117190]

On Wed, Oct 16, 2024 at 11:09:32PM +0200, Jakub Jelinek wrote:
> Apparently my
> c: Speed up compilation of large char array initializers when not using #embed
> patch broke building glibc.
>
> The issue is that when using CPP_EMBED, we are guaranteed by the
> preprocessor that there is CPP_NUMBER CPP_COMMA before it and
> CPP_COMMA CPP_NUMBER after it (or CPP_COMMA CPP_EMBED), so RAW_DATA_CST
> never ends up at the end of arrays of unknown length.
> Now, the c_parser_initval optimization attempted to preserve that property
> rather than changing everything that e.g. inferes array number of elements
> from the initializer etc. to deal with RAW_DATA_CST at the end, but
> it didn't take into account the possibility that there could be
> CPP_COMMA followed by CPP_CLOSE_BRACE (where the CPP_COMMA is redundant).
>
> As we are peaking already at 4 tokens in that code, peeking more would
> require using raw tokens and that seems to be expensive doing it for
> every pair of tokens due to vec_free done when we are out of raw tokens.

Sorry for rushing the previous patch too much, turns out I was wrong,
given that the c_parser_peek_nth_token numbering is 1 based, we can peek
also with c_parser_peek_nth_token (parser, 4) and the loop actually peeked
just at 3 tokens, not 4.

So, I think it is better to revert the previous patch (but keep the new
test) and instead peek the 4th non-raw token, which is what the following
patch does.

Additionally, PR117190 shows one further spot which missed the peek of
the token after CPP_COMMA, in case it is incomplete array with exactly 65
elements with redundant comma after it, which this patch handles too.

2024-10-22  Jakub Jelinek  <jakub@redhat.com>

	PR c/117190
gcc/c/
	* c-parser.cc (c_parser_initval): Revert 2024-10-17 changes.
	Instead peek the 4th token and if it is not CPP_NUMBER,
	handle it like 3rd token CPP_CLOSE_BRACE for orig_len == INT_MAX.
	Also, check (2 + 2 * i)th raw token for the orig_len == INT_MAX
	case and punt if it is not CPP_NUMBER.
gcc/testsuite/
	* c-c++-common/init-5.c: New test.

2 files changed, 35 insertions, 26 deletions

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 090ab1c..3f2d7dd 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -6529,7 +6529,6 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	unsigned int i;
 	gcc_checking_assert (len >= 64);
 	location_t last_loc = UNKNOWN_LOCATION;
-	location_t prev_loc = UNKNOWN_LOCATION;
 	for (i = 0; i < 64; ++i)
 	  {
 	    c_token *tok = c_parser_peek_nth_token_raw (parser, 1 + 2 * i);
@@ -6545,7 +6544,6 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	    buf1[i] = (char) tree_to_uhwi (tok->value);
 	    if (i == 0)
 	      loc = tok->location;
-	    prev_loc = last_loc;
 	    last_loc = tok->location;
 	  }
 	if (i < 64)
@@ -6560,7 +6558,9 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	   (as guaranteed for CPP_EMBED).  */
 	if (tok->type == CPP_CLOSE_BRACE && len != INT_MAX)
 	  len = i;
-	else if (tok->type != CPP_COMMA)
+	else if (tok->type != CPP_COMMA
+		 || (c_parser_peek_nth_token_raw (parser, 2 + 2 * i)->type
+		     != CPP_NUMBER))
 	  {
 	    vals_to_ignore = i;
 	    return;
@@ -6569,7 +6569,6 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	unsigned int max_len = 131072 - offsetof (struct tree_string, str) - 1;
 	unsigned int orig_len = len;
 	unsigned int off = 0, last = 0;
-	unsigned char lastc = 0;
 	if (!wi::neg_p (wi::to_wide (val)) && wi::to_widest (val) <= UCHAR_MAX)
 	  off = 1;
 	len = MIN (len, max_len - off);
@@ -6599,25 +6598,23 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	    if (tok2->type != CPP_COMMA && tok2->type != CPP_CLOSE_BRACE)
 	      break;
 	    buf2[i + off] = (char) tree_to_uhwi (tok->value);
-	    prev_loc = last_loc;
+	    /* If orig_len is INT_MAX, this can be flexible array member and
+	       in that case we need to ensure another element which
+	       for CPP_EMBED is normally guaranteed after it.  Include
+	       that byte in the RAW_DATA_OWNER though, so it can be optimized
+	       later.  */
+	    if (orig_len == INT_MAX
+		&& (tok2->type == CPP_CLOSE_BRACE
+		    || (c_parser_peek_nth_token (parser, 4)->type
+			!= CPP_NUMBER)))
+	      {
+		last = 1;
+		break;
+	      }
 	    last_loc = tok->location;
 	    c_parser_consume_token (parser);
 	    c_parser_consume_token (parser);
 	  }
-	/* If orig_len is INT_MAX, this can be flexible array member and
-	   in that case we need to ensure another element which
-	   for CPP_EMBED is normally guaranteed after it.  Include
-	   that byte in the RAW_DATA_OWNER though, so it can be optimized
-	   later.  */
-	if (orig_len == INT_MAX
-	    && (!c_parser_next_token_is (parser, CPP_COMMA)
-		|| c_parser_peek_2nd_token (parser)->type != CPP_NUMBER))
-	  {
-	    --i;
-	    last = 1;
-	    std::swap (prev_loc, last_loc);
-	    lastc = (unsigned char) buf2[i + off];
-	  }
 	val = make_node (RAW_DATA_CST);
 	TREE_TYPE (val) = integer_type_node;
 	RAW_DATA_LENGTH (val) = i;
@@ -6633,13 +6630,6 @@ c_parser_initval (c_parser *parser, struct c_expr *after,
 	init.original_type = integer_type_node;
 	init.m_decimal = 0;
 	process_init_element (loc, init, false, braced_init_obstack);
-	if (last)
-	  {
-	    init.value = build_int_cst (integer_type_node, lastc);
-	    init.original_code = INTEGER_CST;
-	    set_c_expr_source_range (&init, prev_loc, prev_loc);
-	    process_init_element (prev_loc, init, false, braced_init_obstack);
-	  }
       }
 }
 
diff --git a/gcc/testsuite/c-c++-common/init-5.c b/gcc/testsuite/c-c++-common/init-5.c
new file mode 100644
index 0000000..61b6cdb
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/init-5.c
@@ -0,0 +1,19 @@
+/* PR c/117190 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+struct S { char d[]; } v = {
+{ 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+  0, }
+};
+
+int
+main ()
+{
+  for (int i = 0; i < 65; ++i)
+    if (v.d[i] != (i == 0 ? 8 : 0))
+      __builtin_abort ();
+}