[llvm-bugs] [Bug 36221] New: Support for the mno-stack-arg-probe flag

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36221

            Bug ID: 36221
           Summary: Support for the mno-stack-arg-probe flag
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: nruslan_devel at yahoo.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19802
  --> https://bugs.llvm.org/attachment.cgi?id=19802&action=edit
Patches for clang and LLVM

I created patches (attached) for LLVM and clang that add support for
mno-stack-arg-probe flag. This flag is supported by MinGW/GCC, and it
completely disables stack probes when compiling for PE/COFF (Windows)
executable targets. The patch includes changes for x86 as well as for ARM
(untested), AARCH64 (untested).

Although clang and LLVM already support a related stack-probe-size=<size> flag,
it does not seem to help when the stack usage is unknown at compile time, and
thus probes do not seem to be suppressed even if a very large value is
specified. For example, consider the following code:

extern void func2(void * buffer);

void func(unsigned long size)
{
    char buffer[size];
    func2(buffer);
}

/opt/llvm/bin/clang -Wall -O2 -target x86_64-pc-mingw32 -m64 -S
-mstack-probe-size=2147483647 test.c gives the following output (notice the
___chkstk_ms symbol):

func:                                   # @func
.Lcfi0:
.seh_proc func
# BB#0:
    pushq   %rbp
.Lcfi1:
    .seh_pushreg 5
    movq    %rsp, %rbp
.Lcfi2:
    .seh_setframe 5, 0
.Lcfi3:func:                                   # @func
.Lcfi0:
.seh_proc func
# BB#0:
    pushq   %rbp
.Lcfi1:
    .seh_pushreg 5
    movq    %rsp, %rbp
.Lcfi2:
    .seh_setframe 5, 0
.Lcfi3:func:                                   # @func
.Lcfi0:
.seh_proc func
# BB#0:
    pushq   %rbp
.Lcfi1:
    .seh_pushreg 5
    movq    %rsp, %rbp
.Lcfi2:
    .seh_setframe 5, 0
.Lcfi3:
    .seh_endprologue
    movl    %ecx, %ecx
    addq    $15, %rcx
    movabsq $8589934576, %rax       # imm = 0x1FFFFFFF0
    andq    %rcx, %rax
    callq   ___chkstk_ms
    subq    %rax, %rsp
    movq    %rsp, %rcx
    subq    $32, %rsp
    callq   func2
    nop
    movq    %rbp, %rsp
    popq    %rbp
    retq

With the patch attached, stack probes are no longer used.

/opt/llvm/bin/clang -Wall -O2 -target x86_64-pc-mingw32 -m64 -S
-mno-stack-arg-probe test.c

func:                                   # @func
.Lcfi0:
.seh_proc func
# BB#0:
    pushq   %rbp
.Lcfi1:
    .seh_pushreg 5
    movq    %rsp, %rbp
.Lcfi2:
    .seh_setframe 5, 0
.Lcfi3:
    .seh_endprologue
    movl    %ecx, %eax
    addq    $15, %rax
    movabsq $8589934576, %rax       # imm = 0x1FFFFFFF0
    subq    $-1, %rsp
    movq    %rsp, %rcx
    subq    $32, %rsp
    callq   func2
    nop
    movq    %rbp, %rsp
    popq    %rbp
    retq

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