[Bug tree-optimization/48092] New: associative property of sqrt

["vincenzo.innocente at cern dot ch" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48092

           Summary: associative property of sqrt
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincenzo.innocente@cern.ch


Is there any reason why (with -Ofast or -ffast-math) associative properties of
sqrt are not exploited as for instance those of division?

examples
division (ok)

float div1(float a, float x, float y) {
   return a/x/y;
   0:   f3 0f 59 ca             mulss  %xmm2,%xmm1
   4:   f3 0f 5e c1             divss  %xmm1,%xmm0
}

sqrt

float sqrt1(float a, float x, float y) {
   return a*std::sqrt(x)*std::sqrt(y);
  10:   f3 0f 51 d2             sqrtss %xmm2,%xmm2
  14:   f3 0f 51 c9             sqrtss %xmm1,%xmm1
  18:   f3 0f 59 ca             mulss  %xmm2,%xmm1
  1c:   f3 0f 59 c8             mulss  %xmm0,%xmm1
}
  20:   0f 28 c1                movaps %xmm1,%xmm0

and


float rsqrt1(float a, float x, float y) {
   return a/std::sqrt(x)/std::sqrt(y);
  30:   f3 0f 51 c9             sqrtss %xmm1,%xmm1
  34:   f3 0f 51 d2             sqrtss %xmm2,%xmm2
  38:   f3 0f 59 d1             mulss  %xmm1,%xmm2
  3c:   f3 0f 5e c2             divss  %xmm2,%xmm0
}

in this second case I would have at least expected the use of 
"rsqrtss" to take precedence above the associative property of "div"
emitting the same code as below

float rsqrt2(float a, float x, float y) {
   return a/sqrtf(x*y);
  70:   f3 0f 59 ca             mulss  %xmm2,%xmm1
  74:   f3 0f 52 d9             rsqrtss %xmm1,%xmm3
  78:   f3 0f 59 cb             mulss  %xmm3,%xmm1
  7c:   f3 0f 59 cb             mulss  %xmm3,%xmm1
  80:   f3 0f 59 1d 00 00 00    mulss  0(%rip),%xmm3        # 88 <rsqrt2(float,
float, float)+0x18>
  87:   00 
                        84: R_X86_64_PC32       .LC1+0xfffffffffffffffc
  88:   f3 0f 58 0d 00 00 00    addss  0(%rip),%xmm1        # 90 <rsqrt2(float,
float, float)+0x20>
  8f:   00 
                        8c: R_X86_64_PC32       .LC0+0xfffffffffffffffc
  90:   f3 0f 59 cb             mulss  %xmm3,%xmm1
  94:   f3 0f 59 c8             mulss  %xmm0,%xmm1
}
  98:   0f 28 c1                movaps %xmm1,%xmm0