[Webkit-unassigned] [Bug 54807] New: Always explicitly use "signed" keyword to declare signed bitfields

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=54807

           Summary: Always explicitly use "signed" keyword to declare
                    signed bitfields
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Minor
          Priority: P3
         Component: WebCore Misc.
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: yong.li.webkit at gmail.com


struct {
  int a : 31;
  bool b : 1;
};

Many compilers will treat "a" as a signed integer. However, according to standards, making "a" signed or unsigned relies on the implementation of the compiler. We have known armcc treats "a" as unsigned integer. (See Bug 54687). This difference can cause unexpected runtime error. For example:

a = -1;
if (a == -1)
  printf("a is signed");
else
  printf("a is unsigned"); // -1 will be converted to unsigned number first => 0xFFFFFFFF, and "a" is 0x7FFFFFFF, so they are different.

To make sure our code has same behavior for all compilers and avoid hard-to-locate bugs caused by this, we should always explicitly use "signed" keywords when we want to use signed bitfields (and of course, use "unsigned" keyword in the other case), given that the change will be no-op for most compilers.

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