From 3e1952ed96dde926ce38a8ab8803f9a410ecdae3 Mon Sep 17 00:00:00 2001
From: Jeff King <peff@peff.net>
Date: Tue, 30 Aug 2016 23:43:07 -0400
Subject: [PATCH] color_parse_mem: initialize "struct color" temporary
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Compiling color.c with gcc 6.2.0 using -O3 produces some
-Wmaybe-uninitialized false positives:

    color.c: In function ‘color_parse_mem’:
    color.c:189:10: warning: ‘bg.blue’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       out += xsnprintf(out, len, "%c8;2;%d;%d;%d", type,
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          c->red, c->green, c->blue);
          ~~~~~~~~~~~~~~~~~~~~~~~~~~
    color.c:208:15: note: ‘bg.blue’ was declared here
      struct color bg = { COLOR_UNSPECIFIED };
                   ^~
    [ditto for bg.green, bg.red, fg.blue, etc]

This is doubly confusing, because the declaration shows it
being initialized! Even though we do not explicitly
initialize the color components, an incomplete initializer
sets the unmentioned members to zero.

What the warning doesn't show is that we later do this:

  struct color c;
  if (!parse_color(&c, ...)) {
          if (fg.type == COLOR_UNSPECIFIED)
                fg = c;
          ...
  }

gcc is clever enough to realize that a struct assignment
from an uninitialized variable taints the destination. But
unfortunately it's _not_ clever enough to realize that we
only look at those members when type is set to COLOR_RGB, in
which case they are always initialized.

With -O2, gcc does not look into parse_color() and must
assume that "c" emerges fully initialized. With -O3, it
inlines parse_color(), and learns just enough to get
confused.

We can silence the false positive by initializing the
temporary "c". This also future-proofs us against
violating the type assumptions (the result would probably
still be buggy, but in a deterministic way).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 color.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/color.c b/color.c
index 8f85153d0d..c809548991 100644
--- a/color.c
+++ b/color.c
@@ -200,7 +200,7 @@ int color_parse_mem(const char *value, int value_len, char *dst)
 	/* [fg [bg]] [attr]... */
 	while (len > 0) {
 		const char *word = ptr;
-		struct color c;
+		struct color c = { COLOR_UNSPECIFIED };
 		int val, wordlen = 0;
 
 		while (len > 0 && !isspace(word[wordlen])) {