As the answer on stackoverflow mentions, there is -ffast-math option to gcc that tells the compiler to treat fp operations as associative. I've done some testing:

  $ clang -v
  clang version 3.3 (tags/RELEASE_33/final)
  Target: x86_64-apple-darwin12.4.0

  double pow(double);
  
        LLVM bitcode                   Assembly

  // a*a*a*a*a*a                 
  %1 = fmul double %a, %a       |  movaps %xmm0, %xmm1
  %2 = fmul double %1, %a       |  mulsd  %xmm1, %xmm1
  %3 = fmul double %2, %a       |  mulsd  %xmm0, %xmm1
  %4 = fmul double %3, %a       |  mulsd  %xmm0, %xmm1
  %5 = fmul double %4, %a       |  mulsd  %xmm0, %xmm1
  ret double %5                 |  mulsd  %xmm0, %xmm1
                                |  movaps %xmm1, %xmm0

  // a*a*a*a*a*a -ffast-math    
  %1 = fmul fast double %a, %a  |  mulsd  %xmm0, %xmm0
  %2 = fmul fast double %1, %1  |  movaps %xmm0, %xmm1
  %3 = fmul fast double %1, %2  |  mulsd  %xmm1, %xmm1
  ret double %3                 |  mulsd  %xmm0, %xmm1
                                |  movaps %xmm1, %xmm0

  // (a*a*a)*(a*a*a)            
  %1 = fmul fast double %a, %a  |  movaps %xmm0, %xmm1
  %2 = fmul fast double %1, %a  |  mulsd  %xmm0, %xmm0
  %3 = fmul fast double %2, %2  |  mulsd  %xmm1, %xmm0
  ret double %3                 |  mulsd  %xmm0, %xmm0