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GCC: Specifying mmx register for a local variable fails

0

Im trying to achieve the following on a Windows environment c++, MINGW GCC:

Im trying to define a local variable as such:

register DWORD index __asm ("%mm7");

this definition compiles with MINGW GCC x64 , but fails on x86 compilation with the error
error: invalid register name for 'index'
register DWORD index __asm ("%mm7");

I have already tried to reference the register as __asm ("mm7"); but to no avail. any help on the matter would be very appreciated

c++ assembly x86 g++ mingw

share|improve this question

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I'm pretty sure you can only use general purpose registers with this feature. What are you trying to do?
  
  – fuz
  Mar 6 at 14:29
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Whats weird is that it does succeed on a x64 toolchain compilation. im trying to save the value in this register to be referenced later by a different func, by accessing the register
  
  – win32api
  Mar 6 at 14:31
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Try specifying -march=native or -mmmx.
  
  – Maxim Egorushkin
  Mar 6 at 16:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I'd also question how this is being used.It should be noted that using register DWORD index __asm ("%mm7"); doesn't guarantee the variable index would be in mm7. The only guarantee is that if index was used as an operand to inline assembly using a "y" constraint, mm7 would be the register used. As @MaximEgorushkin points out to even get this going you'd have to tell the compiler to use MMX via the -mmmx option. x86-64 should compile without error b/c all x86-64 processors can use MMX registers,but not all x86 targets support MMX so the compiler has to be told to use them or not.
  
  – Michael Petch
  Mar 6 at 18:34
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Compiling without error and generating the code you expect are two different issues.
  
  – Michael Petch
  Mar 6 at 18:35
  

  
  
  
  

 | 
show 4 more comments

0

Im trying to achieve the following on a Windows environment c++, MINGW GCC:

Im trying to define a local variable as such:

register DWORD index __asm ("%mm7");

this definition compiles with MINGW GCC x64 , but fails on x86 compilation with the error
error: invalid register name for 'index'
register DWORD index __asm ("%mm7");

I have already tried to reference the register as __asm ("mm7"); but to no avail. any help on the matter would be very appreciated

c++ assembly x86 g++ mingw

share|improve this question

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I'm pretty sure you can only use general purpose registers with this feature. What are you trying to do?
  
  – fuz
  Mar 6 at 14:29
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Whats weird is that it does succeed on a x64 toolchain compilation. im trying to save the value in this register to be referenced later by a different func, by accessing the register
  
  – win32api
  Mar 6 at 14:31
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Try specifying -march=native or -mmmx.
  
  – Maxim Egorushkin
  Mar 6 at 16:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I'd also question how this is being used.It should be noted that using register DWORD index __asm ("%mm7"); doesn't guarantee the variable index would be in mm7. The only guarantee is that if index was used as an operand to inline assembly using a "y" constraint, mm7 would be the register used. As @MaximEgorushkin points out to even get this going you'd have to tell the compiler to use MMX via the -mmmx option. x86-64 should compile without error b/c all x86-64 processors can use MMX registers,but not all x86 targets support MMX so the compiler has to be told to use them or not.
  
  – Michael Petch
  Mar 6 at 18:34
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Compiling without error and generating the code you expect are two different issues.
  
  – Michael Petch
  Mar 6 at 18:35
  

  
  
  
  

 | 
show 4 more comments

Im trying to achieve the following on a Windows environment c++, MINGW GCC:

Im trying to define a local variable as such:

register DWORD index __asm ("%mm7");

this definition compiles with MINGW GCC x64 , but fails on x86 compilation with the error
error: invalid register name for 'index'
register DWORD index __asm ("%mm7");

I have already tried to reference the register as __asm ("mm7"); but to no avail. any help on the matter would be very appreciated

c++ assembly x86 g++ mingw

share|improve this question

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited Mar 6 at 23:22

Michael Petch

26.5k556105

edited Mar 6 at 23:22

Michael Petch

26.5k556105

asked Mar 6 at 14:24

win32apiwin32api

1

New contributor

win32api is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked Mar 6 at 14:24

win32apiwin32api

1