customised op using MULT instruction

Hi,

I am trying to make a customized operation, where the result of MULT
operation is used in adder or subtractor, e.g.: result = ( ( (a) *
(b) ) - (c) )

However, compiler just cannot recognise the pattern (: reg:
SUBI4(MULTI4(reg,reg), reg ) or reg: SUBU4(MULTU4(reg, reg ) )

Is there another way that customised op usingmultiply instruction
should be  made ? The following works fine, where the MULT uses the
result of SUB : ( ( (a) * (( b ) - (c ) )

Please suggest ! Particularly, I am interested in identifying the
pattern : result = (( ( (a) * (b) ) - ( (c) * (d) ) )

Thanks,
Alok Sharma