Bug 49816 – Remove a couple unneeded overflow checks

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKit Misc.

[Darin Adler]

Remove a couple unneeded overflow checks

[Darin Adler]

Created attachment 74408 [details]
Patch

[Andreas Kling]

Comment on attachment 74408 [details]
Patch

r=me

[Alexey Proskuryakov]

+    // We need to be sure we can double the length without overflowing.
+    // Since the passed-in length is the length of an actual existing
+    // string, and we can guarantee that doesn't occupy the entire
+    // address space, we can just assert here and there's no need for
+    // a runtime check.
+    ASSERT(length <= numeric_limits<size_t>::max() / 2);

This comment isn't sufficient for me to understand. If the existing string takes 51% of address space, why doesn't doubling that cause an overflow?

[Andreas Kling]

(In reply to comment #3)
> This comment isn't sufficient for me to understand. If the existing string takes 51% of address space, why doesn't doubling that cause an overflow?

The _existing_ string is length*2 bytes large, so we know that length*2 fits in the address space.

[Darin Adler]

(In reply to comment #3)
> +    // We need to be sure we can double the length without overflowing.
> +    // Since the passed-in length is the length of an actual existing
> +    // string, and we can guarantee that doesn't occupy the entire
> +    // address space, we can just assert here and there's no need for
> +    // a runtime check.
> +    ASSERT(length <= numeric_limits<size_t>::max() / 2);
> 
> This comment isn't sufficient for me to understand. If the existing string takes 51% of address space, why doesn't doubling that cause an overflow?

If the string’s length was the same as 51% of the address space, the string’s size would be 102% of the address space, since characters are two bytes each. I should refine the comment to make this clearer.

[Darin Adler]

Committed r72831: <http://trac.webkit.org/changeset/72831>