do not depend on signed integer overflow

Signed integer overflow is not defined in C, so do not depend on it.

This fixes a problem with GCC 4.4.0 and -O3 where the optimizer would
consider "consumed_bytes > consumed_bytes + bytes" as a constant
expression, and never execute the die()-call.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 2e680d7..e243d9d 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -161,7 +161,7 @@ static void use(int bytes)
        input_offset += bytes;
 
        /* make sure off_t is sufficiently large not to wrap */
-       if (consumed_bytes > consumed_bytes + bytes)
+       if (signed_add_overflows(consumed_bytes, bytes))
                die("pack too large for current definition of off_t");
        consumed_bytes += bytes;
 }

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 3756cf3..d5a8db1 100644 (file)
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -431,7 +431,7 @@ static int write_one(struct sha1file *f,
        written_list[nr_written++] = &e->idx;
 
        /* make sure off_t is sufficiently large not to wrap */
-       if (*offset > *offset + size)
+       if (signed_add_overflows(*offset, size))
                die("pack too large for current definition of off_t");
        *offset += size;
        return 1;

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 685566e..f63973c 100644 (file)
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -83,7 +83,7 @@ static void use(int bytes)
        offset += bytes;
 
        /* make sure off_t is sufficiently large not to wrap */
-       if (consumed_bytes > consumed_bytes + bytes)
+       if (signed_add_overflows(consumed_bytes, bytes))
                die("pack too large for current definition of off_t");
        consumed_bytes += bytes;
 }

diff --git a/git-compat-util.h b/git-compat-util.h
index 81883e7..2af8d3e 100644 (file)
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -28,6 +28,18 @@
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 #define bitsizeof(x)  (CHAR_BIT * sizeof(x))
 
+#define maximum_signed_value_of_type(a) \
+    (INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))
+
+/*
+ * Signed integer overflow is undefined in C, so here's a helper macro
+ * to detect if the sum of two integers will overflow.
+ *
+ * Requires: a >= 0, typeof(a) equals typeof(b)
+ */
+#define signed_add_overflows(a, b) \
+    ((b) > maximum_signed_value_of_type(a) - (a))
+
 #ifdef __GNUC__
 #define TYPEOF(x) (__typeof__(x))
 #else