[Bug rtl-optimization/17236] inefficient code for long long multiply on x86

[bonzini at gnu dot org]

------- Comment #23 from bonzini at gnu dot org  2007-12-19 13:36 -------
Created an attachment (id=14794)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14794&action=view)
teach combine that reg = op(reg, mem) is better

Since combine operates on the whole pattern, it can be taught the trick that
swap_commutative_operands_with_target does in optabs.c.  This is the "right
way" to do the hack Uros proposed in comment #18.

Note that optabs.c does not get it because it creates reg=subreg*subreg.  Then
CSE creates SImode memory loads from the subreg, and combine creates
reg=mem*reg.  With this patch, combine creates reg=reg*mem instead.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17236