[build] Disable strict-overflow check on gcc

This flag generates false positives, since gcc inlines functions and
propagates constants, and then applies the check.

Drive-by: Refactor the checks that triggered the error to avoid
explicit casts.

R=jochen@chromium.org, machenbach@chromium.org
BUG=v8:6341

Change-Id: I86aebf402cbd2502ef17622a000a5bb777fd4b43
Reviewed-on: https://chromium-review.googlesource.com/494474Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45096}

BUILD.gn

@@ -480,6 +480,15 @@ config("toolchain") {
       "/wd4800",  # Forcing value to bool.
     ]
   }
+
+  if (!is_clang && !is_win) {
+    cflags += [
+      # Disable gcc warnings for optimizations based on the assumption that
+      # signed overflow does not occur. Generates false positives (see
+      # http://crbug.com/v8/6341).
+      "-Wno-strict-overflow",
+    ]
+  }
 }
 
 ###############################################################################

gypfiles/standalone.gypi

@@ -780,6 +780,12 @@
               # Don't warn about unrecognized command line option.
               '-Wno-gnu-zero-variadic-macro-arguments',
             ],
+            'cflags' : [
+              # Disable gcc warnings for optimizations based on the assumption
+              # that signed overflow does not occur. Generates false positives
+              # (see http://crbug.com/v8/6341).
+              "-Wno-strict-overflow",
+            ],
           }],
           [ 'clang==1 and (v8_target_arch=="x64" or v8_target_arch=="arm64" \
             or v8_target_arch=="mips64el")', {

src/vector.h

@@ -34,9 +34,9 @@ class Vector {
   // Returns a vector using the same backing storage as this one,
   // spanning from and including 'from', to but not including 'to'.
   Vector<T> SubVector(int from, int to) const {
-    DCHECK(0 <= from);
-    SLOW_DCHECK(from <= to);
-    SLOW_DCHECK(static_cast<unsigned>(to) <= static_cast<unsigned>(length_));
+    DCHECK_LE(0, from);
+    DCHECK_LE(from, to);
+    DCHECK_LE(to, length_);
     return Vector<T>(start() + from, to - from);
   }