Re: [PATCH] MdePkg/BaseSynchronizationLib: fix XADD operands in GCC IA32/X64 assembly

[Laszlo Ersek <lersek@redhat.com>]

Hi,

On 09/25/18 21:48, Laszlo Ersek wrote:
> Currently, "gcc-4.8.5-28.el7_5.1.x86_64" generates the following code for
> me, from the XADD inline assembly added to "X64/GccInline.c" in commit
> 17634d026f96:
> 
>> 0000000000004383 <InternalSyncIncrement>:
>> UINT32
>> EFIAPI
>> InternalSyncIncrement (
>>   IN      volatile UINT32    *Value
>>   )
>> {
>>     4383:       55                      push   %rbp
>>     4384:       48 89 e5                mov    %rsp,%rbp
>>     4387:       48 83 ec 10             sub    $0x10,%rsp
>>     438b:       48 89 4d 10             mov    %rcx,0x10(%rbp)
>>   UINT32  Result;
>>
>>   __asm__ __volatile__ (
>>     438f:       48 8b 55 10             mov    0x10(%rbp),%rdx
>>     4393:       48 8b 45 10             mov    0x10(%rbp),%rax
>>     4397:       b8 01 00 00 00          mov    $0x1,%eax
>>     439c:       f0 0f c1 00             lock xadd %eax,(%rax)
>>     43a0:       ff c0                   inc    %eax
>>     43a2:       89 45 fc                mov    %eax,-0x4(%rbp)
>>     : "m"  (*Value)           // %2
>>     : "memory",
>>       "cc"
>>     );
>>
>>   return Result;
>>     43a5:       8b 45 fc                mov    -0x4(%rbp),%eax
>> }
>>     43a8:       c9                      leaveq
>>     43a9:       c3                      retq
>>
> 
> The MOV $0X1,%EAX instruction corrupts the address of Value in %RAX before
> we reach the XADD instruction. In fact, it makes no sense for XADD to use
> %EAX as source operand and (%RAX) as destination operand at the same time.

may I get a fast review for this patch, please? The regression from
commit 17634d026f96 prevents OVMF from booting.

Thanks
Laszlo