[LLVMbugs] [Bug 21895] New: LLVM hardcodes stack probe space to 4096 bytes

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21895

            Bug ID: 21895
           Summary: LLVM hardcodes stack probe space to 4096 bytes
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: clangbugs.10.mdwxzxkl at spamgourmet.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The LLVM stack probe space (e.g. used for issuing __chkstk() calls when
compiling to Windows targets) is hard coded to 4096 bytes. However, in order to
suppress __chkstk() calls, it should be possible to specify a minimum value for
issuing stack probe calls that is greater than 4096 bytes. 

This is needed in order to properly implement the /Gs switch of the MSVC
compiler. 

The /Gs compiler switch is needed to suppress calls to __chkstk (the stack
probing function used by MSVC), e.g. when one carefully uses stack memory so
that all necessary stack probes are initiated, or when compiling with
/NODEFAULTLIB (as __chkstk would then be undefined). 

Reference to the hardcoded value of 4096 bytes:
http://llvm.org/klaus/llvm/blob/master/lib/Target/X86/X86FrameLowering.cpp#L-709

Reference:
http://msdn.microsoft.com/en-us/library/9598wk25

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