GDC optimizations bug ?

[Maor via D.gnu]

Hi,

I'm trying to compile a program using inline asm with 
optimizations and I got my inline asm functions thrown out by the 
optimizer although I declared them as having side effects (by 
stating a memory clobber).

I wrote the following toy program to demonstrate the problem:

--

import std.stdio;
import gcc.attribute;

@attribute("forceinline") ulong exchangeAndAdd(shared ulong 
*counter, ulong addition) {
  ulong retVal = void; // we don't want it initialized when 
dmd is used

  asm {
"
  mov  %2, %0 ;
  lock;
  xadd %0, (%1)   ;
":
"=&r"(retVal) :
"r"(counter), "r"(addition) :
"memory";
  }
  return retVal;
}

ulong func1() {
  shared ulong a = 9;
  exchangeAndAdd(&a, 3);
  return a;
}

void main()
{
  ulong b;
  b = func1();
  writeln(b);
}

--

Compiling it with and without optimizations gives a different 
output:

/opt/gdc/bin/gdc ./test.d -o/tmp/a.out
/tmp/a.out

12

/opt/gdc/bin/gdc -O3 -g ./test.d -o/tmp/a.out
/tmp/a.out

9

using gdb we can see that the call to exchangeAndAdd() is being 
thrown out in the optimized version:


(gdb) disas _D4test5func1FZm
Dump of assembler code for function _D4test5func1FZm:
   0x00406460 <+0>:   movq   $0x9,-0x8(%rsp)
   0x00406469 <+9>:   mov-0x8(%rsp),%rax
   0x0040646e <+14>:  retq
End of assembler dump.


Is this a compiler bug or am I doing something wrong ?
btw, I'm using gdc 4.9.0.

thanks in advance,
Maor

Re: GDC optimizations bug ?

[Maor via D.gnu]

On Sunday, 9 November 2014 at 11:50:24 UTC, Iain Buclaw via D.gnu 
wrote:
On 9 November 2014 08:54, Iain Buclaw  
wrote:


On 9 Nov 2014 08:40, "Maor via D.gnu"  
wrote:


Hi,

I'm trying to compile a program using inline asm with 
optimizations and I
got my inline asm functions thrown out by the optimizer 
although I declared

them as having side effects (by stating a memory clobber).
I wrote the following toy program to demonstrate the problem:

--

import std.stdio;
import gcc.attribute;

@attribute("forceinline") ulong exchangeAndAdd(shared ulong 
*counter,

ulong addition) {
  ulong retVal = void; // we don't want it initialized 
when dmd is

used
  asm {
"
  mov  %2, %0 ;
  lock;
  xadd %0, (%1)   ;
":
"=&r"(retVal) :
"r"(counter), "r"(addition) :


Maybe try:  "=m"(*counter)

The bug is likely in your input/output clobbers, gcc will 
always optimise
against you unless you get the input/output/clobbers precisely 
correct.


Yep, it looks like using (%1) -> "r"(counter) creates a 
temporary in
the register but never binds back.  The optimiser sees that 
memory is

clobbered, but non of the operands have memory side effects.

Telling gcc that this is a memory operand fixes it - %1 ->
"m"(counter).  But I think that having counter as an input 
operand is
wrong, as it *has* infact a new value written to it.  You can 
also
omit the 'mov' instruction by telling gcc the "0" register 
should be

loaded with the input "addition"

Your fixed (and simplified) assembler statement now becomes:


@attribute("forceinline") ulong exchangeAndAdd(shared ulong 
*counter,

ulong addition) {
  ulong retVal = void; // we don't want it initialized when 
dmd is used

  asm {
  "
  lock;
  xadd %0, %1 ;
  " :
  "=r"(retVal), "=m"(*counter) :
  "0"(addition) :
  "memory";
  }
  return retVal;
}


Hi,

Thanks for the tip!
Indeed, it solves the problem (which uncovered another one, but 
one which deserves a different subject :)


Cheers,
Maor