Re: [edk2-devel] question about MSFT compiler/link option

["Andrew Fish" <afish@apple.com>]

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> On Aug 5, 2020, at 8:31 PM, Tiger Liu(BJ-RD) <tigerliu@zhaoxin.com> wrote:
> 
> Hi, Michael:
> I wrote this sentence in C file:
> pTmp = (VOID*)& PdataStructTable;
> 

Tiger,

Working around optimization is complicated. You likely added a statement that got optimized away as the value of pTmp could be ignored and your program could still get the same result. 

You would need to do some meaningful work with pTmp or make pTmp a volatile variable to force the compiler to NOT optimize it away. The better the optimizer, the more layers of abstraction it can see and optimize away. 

Thanks,

Andrew Fish

> tried to reference the table being generated to prevent the optimizer from removing the data structure from the executable.
> 
> But it is still being optimized away.
> 
> I used MSTF VS2015.
> 
> Thanks
> -----邮件原件-----
> 发件人: devel@edk2.groups.io <mailto:devel@edk2.groups.io> <devel@edk2.groups.io <mailto:devel@edk2.groups.io>> 代表 Michael D Kinney
> 发送时间: 2020年8月5日 23:18
> 收件人: devel@edk2.groups.io <mailto:devel@edk2.groups.io>; Tiger Liu(BJ-RD) <TigerLiu@zhaoxin.com <mailto:TigerLiu@zhaoxin.com>>; Gao, Liming <liming.gao@intel.com <mailto:liming.gao@intel.com>>; Kinney, Michael D <michael.d.kinney@intel.com <mailto:michael.d.kinney@intel.com>>
> 主题: Re: [edk2-devel] question about MSFT compiler/link option
> 
> Hi,
> 
> I think you are seeing the impact of compiler optimizations.
> 
> If a global variable is never referenced after optimizing the code, then the global variable will be optimized away.  This type of optimization is important for size constrained environment like firmware.
> 
> By using the /Gw- flag, you are disabling the ability of the compiler to optimize away unreferenced global variables.
> 
> If you update the code in your example to guarantee the global variable is always accessed for all types of builds, then I suspect you will always see the global in the map.
> 
> Thanks,
> 
> Mike
> 
>> -----Original Message-----
>> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Tiger
>> Liu(BJ-RD)
>> Sent: Tuesday, August 4, 2020 11:34 PM
>> To: devel@edk2.groups.io; Gao, Liming
>> <liming.gao@intel.com>
>> Subject: Re: [edk2-devel] question about MSFT compiler/link option
>> 
>> Hi, Liming:
>> Thanks for your reply!
>> 
>> I checked map file, not find this global data array.
>> 
>> I found if added this build option in this PEIM's inf, then it seems
>> ok.
>> [BuildOptions]
>> MSFT:*_*_IA32_CC_FLAGS   = /Gw-
>> 
>> Thanks
>> 
>> -----邮件原件-----
>> 发件人: devel@edk2.groups.io <devel@edk2.groups.io> 代表
>> Liming Gao
>> 发送时间: 2020年8月4日 23:21
>> 收件人: devel@edk2.groups.io; Tiger Liu(BJ-RD) <TigerLiu@zhaoxin.com>
>> 主题: Re: [edk2-devel] question about MSFT compiler/link option
>> 
>> If this global variable is used by code, it will be linked into the
>> final EFI image. You can check whether the generate map file includes
>> it.
>> 
>>> -----Original Message-----
>>> From: devel@edk2.groups.io <devel@edk2.groups.io> On
>> Behalf Of Tiger
>>> Liu(BJ-RD)
>>> Sent: Tuesday, August 4, 2020 5:34 PM
>>> To: devel@edk2.groups.io
>>> Subject: [edk2-devel] question about MSFT
>> compiler/link option
>>> 
>>> Hi, Experts:
>>> I have a question about MSFT compiler/link option.
>>> 
>>> I wrote a PEIM, which had a large global data array
>> definition in a C file.
>>> Such as:
>>> PRI_DATA_STRUCT  PdataStructTable[] = {
>>> {01,           0x75,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> {02,           0x77,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> {03,           0x79,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> {04,           0x7B,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> {05,           0x7D,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> {06,           0x7F,    0xF0,     0x0,     0x0,
>> 0x0 },
>>> ......
>>> };
>>> 
>>> This global data array is very large, about 20KB.
>>> 
>>> But I found the PEIM's compiled size is about 5KB, it
>> seems not include this global data array in the efi binary statically.
>>> Even the release version PEIM's size is larger than
>> debug version.
>>> 
>>> I found when compiling release version, the link
>> option has :
>>> /MERGE:.rdata=.data It seems release binary included
>> the global data array in the final efi image.
>>> 
>>> So, my question is:
>>> Will the global data array be in linked into the final
>> efi binary image when compiling debug version?
>>> 
>>> Thanks
>>> 
>>> 
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> 
> 
> 
> 
> 
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