[llvm-bugs] [Bug 38679] New: -GS stack buffer checks are expensive

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38679

            Bug ID: 38679
           Summary: -GS stack buffer checks are expensive
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dmajor at mozilla.com
                CC: froydnj at gmail.com, hans at chromium.org,
                    llvm-bugs at lists.llvm.org, rnk at google.com

We recently observed that compiling Firefox with `-GS-` gives a 4-6%
improvement to our perf test suite in clang-cl builds. Adding the same flag to
MSVC builds gives a <1% improvement if anything.

Is there anything that can be done to improve clang's implementation? I found
this review (a bit old, not sure if it's still relevant)
https://reviews.llvm.org/D20346 that says "I think we can look into situations
where Clang emits stack guards and MSVC doesn't if that turns out to be a
problem later."

As a first guess:
https://msdn.microsoft.com/en-us/library/8dbf701c.aspx#Anchor_3 describes some
limitations on the buffers that MSVC checks. I wonder if clang that into
account?

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