Hi,
I am trying to see if moving mostly unused code (e.g. conditional
debug print statements) to a different section (and to different
pages) would impact performance in a large application.
Thanks,
Saul
On Wed, Feb 19, 2014 at 9:11 AM, Ian Lance Taylor <iant@xxxxxxxxxx> wrote:
> On Wed, Feb 19, 2014 at 5:57 AM, Saul Tamari <stamari@xxxxxxxxx> wrote:
>>
>> I'm trying to move some C++ code to a different ELF section and am
>> facing some errors which I don't understand. I'm using g++ v4.8.1 on
>> x86.
>>
>> When compiling the following code I'm getting these errors:
>> /tmp/ccpp2AkE.s: Assembler messages:
>> /tmp/ccpp2AkE.s:63: Error: CFI instruction used without previous .cfi_startproc
>> /tmp/ccpp2AkE.s:64: Error: CFI instruction used without previous .cfi_startproc
>> /tmp/ccpp2AkE.s:66: Error: .cfi_endproc without corresponding .cfi_startproc
>> /tmp/ccpp2AkE.s: Error: open CFI at the end of file; missing
>> .cfi_endproc directive
>>
>>
>> The source is:
>> #include <iostream>
>> #include <stdlib.h>
>>
>> int qqq;
>>
>> int main(int argc, char* argv[])
>> {
>> std::cout << "hey " << std::endl;
>>
>> qqq = rand();
>> if (qqq > 0x1000000) {
>> asm volatile ("jmp 1f \n\t .pushsection
>> __kuku,\"ax\",@progbits \n\t 1:");
>> std::cout << "0x123456" << std::endl;
>> throw 12345;
>> asm volatile("jmp 3f \n\t .popsection \n\t 3:");
>> }
>>
>> return 0;
>> }
>>
>>
>> What do these errors mean? Is there a way to fix them? Is there an
>> alternate method to move similar code to a different section?
>
> The assembler errors occur because GCC emits debug info in the
> assembler stream using CFI pseudo-ops, and you are moving the
> pseudo-ops to a different section in a way that GCC does not
> understand. The assembler is seeing CFI pseudo-ops that make no
> sense, so it is giving errors about them.
>
> The approach you are using can not work. The compiler is not an
> assembler. It does not issue instructions in precise sequence. It
> copies and duplicates and rearranges instructions as it sees fit.
> This is so even though you are using asm volatile. All the asm
> volatile promises is that the string will appear at the right point in
> execution sequence. Your strings can only work if they appear at the
> right point in the assembler output. That is a different matter that
> the compiler does not guarantee.
>
> You didn't see what you are trying to do, but at a guess you should
> look at the -freorder-blocks-and-partition option.
>
> Ian
