Re: [PATCH 3/3] MdePkg/BaseSynchronizationLib: Remove inline X86 assembly code

["Gao, Liming" <liming.gao@intel.com>]

Andrew:
  I want to keep only one implementation. If inline assembly c source is preferred, I suggest to remove its nasm implementation.

Thanks
Liming
From: afish@apple.com [mailto:afish@apple.com]
Sent: Tuesday, March 5, 2019 2:44 PM
To: Gao, Liming <liming.gao@intel.com>
Cc: Zhang, Shenglei <shenglei.zhang@intel.com>; edk2-devel-01 <edk2-devel@lists.01.org>; Kinney, Michael D <michael.d.kinney@intel.com>
Subject: Re: [edk2] [PATCH 3/3] MdePkg/BaseSynchronizationLib: Remove inline X86 assembly code




On Mar 5, 2019, at 1:32 PM, Gao, Liming <liming.gao@intel.com<mailto:liming.gao@intel.com>> wrote:

Andrew:
 BZ 1163 is to remove inline X86 assembly code in C source file. But, this patch is wrong. I have gave my comments to update this patch.


Why do we want to remove inline X86 assembly. As I mentioned it will lead to larger binaries, slower binaries, and less optimized code.

For example take this simple inline assembly function:

VOID
EFIAPI
CpuBreakpoint (
  VOID
  )
{
  __asm__ __volatile__ ("int $3");
}


Today with clang LTO turned on calling CpuBreakpoint() looks like this from the callers point of view.

a.out[0x1fa5] <+6>:  cc              int3


But if we move that to NASM:


a.out[0x1fa6] <+7>:  e8 07 00 00 00  calll  0x1fb2                    ; CpuBreakpoint


plus:
a.out`CpuBreakpoint:
a.out[0x1fb2] <+0>: cc     int3
a.out[0x1fb3] <+1>: c3     retl


And there is also an extra push and pop on the stack. The other issue is the call to the function that is unknown to the compiler will act like a _ReadWriteBarrier (Yikes I see _ReadWriteBarrier is deprecated in VC++ 2017). Is the depreciation of some of these intrinsics in VC++ driving the removal of inline assembly? For GCC inline assembly works great for local compile, and for clang LTO it works in entire program optimization.

The LTO bitcode includes inline assembly and constraints so that the optimizer knows what to do so it can get optimized just like the abstract bitcode during the Link Time Optimization.

This is CpuBreakpoint():
; Function Attrs: noinline nounwind optnone ssp uwtable
define void @CpuBreakpoint() #0 {
  call void asm sideeffect "int $$3", "~{dirflag},~{fpsr},~{flags}"() #2, !srcloc !3
  ret void
}


This is AsmReadMsr64():
; Function Attrs: noinline nounwind optnone ssp uwtable
define i64 @AsmReadMsr64(i32) #0 {
  %2 = alloca i32, align 4
  %3 = alloca i64, align 8
  store i32 %0, i32* %2, align 4
  %4 = load i32, i32* %2, align 4
  %5 = call i64 asm sideeffect "rdmsr", "=A,{cx},~{dirflag},~{fpsr},~{flags}"(i32 %4) #2, !srcloc !4
  store i64 %5, i64* %3, align 8
  %6 = load i64, i64* %3, align 8
  ret i64 %6
}



I agree it make sense to remove .S and .asm files and only have .nasm files.

Thanks,

Andrew Fish

PS For the Xcode clang since it emits frame pointers by default we need to add the extra 4 bytes to save the assembly functions so the stack can get unwound.

a.out`CpuBreakpoint:
a.out[0x1f99] <+0>: 55     pushl  %ebp
a.out[0x1f9a] <+1>: 89 e5  movl   %esp, %ebp
a.out[0x1f9c] <+3>: cc     int3
a.out[0x1f9d] <+4>: 5d     popl   %ebp
a.out[0x1f9e] <+5>: c3     retl


So breakpoint goes from 1 byte to 11 bytes if we get rid of the intrinsics.


 The change is to reduce the duplicated implementation. It will be good on the code maintain. Recently, one patch has to update .c and .nasm both. Please see https://lists.01.org/pipermail/edk2-devel/2019-February/037165.html. Beside this change, I propose to remove all GAS assembly code for IA32 and X64 arch. After those change, the patch owner will collect the impact of the image size.

Thanks
Liming