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246 x87 Floating-Point Optimizations Chapter 10
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
10.5 Floating-Point Subexpression Elimination
Optimization
Reduce the number of superfluous FXCH instructions by putting the shared source operand at the top
of the stack to eliminate subexpressions.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
There are cases that do not require an FXCH instruction after every instruction to allow access to two
new stack entries. In the cases where two instructions share a source operand, an FXCH is not
required between the two instructions. When there is an opportunity for subexpression elimination,
reduce the number of superfluous FXCH instructions by putting the shared source operand at the top
of the stack—for example:
Examples
Listing 29. Avoid
;=====================
; func((x*y),(x+z))
;=====================
fld x ; x
fld y ; y x
fld x ; x y x
fld z ; z x y x
faddp st(1), st ; x+z y x
fxch st(2) ; x y x+z
fmulp st(1), st ; x*y x+z
Listing 30. Preferred
fld z ; z
fld y ; y z
fld x ; x y z
fmul st(1), st ; x x*y z
faddp st(2), st ; x*y x+z