[PATCH] D141180: [X86] Only match BMI (BLSR, BLSI, BLSMSK) if the add/sub op is single use

[Phoebe Wang via Phabricator via llvm-commits]

pengfei added inline comments.


================
Comment at: llvm/lib/Target/X86/X86InstrInfo.td:1272
 
+// An 'ineg' node with a single use.
+def ineg_su : PatFrag<(ops node:$v), (ineg node:$v), [{
----------------
goldstein.w.n wrote:
> craig.topper wrote:
> > Maybe we should have
> > 
> > ```
> > `class binop_oneuse<SDPatternOperator operator>
> >     : PatFrag<(ops node:$A, node:$B),
> >               (operator node:$A, node:$B), [{
> >   return N->hasOneUse();
> > }]>;`
> > 
> > def add_su : binop_oneuse<add>;
> > def and_su : binop_oneuse<and>;
> > def srl_su : binop_oneuse<srl>;
> > 
> > class unop_oneuse<SDPatternOperator operator>
> >     : PatFrag<(ops node:$A),
> >               (operator node:$A), [{
> >   return N->hasOneUse();
> > }]>;
> > 
> > def ineg_su : unop_oneuse<ineg>;
> > def trunc_su : unop_oneuse<trunc>;
> > ```
> > Maybe we should have
> > 
> > ```
> > `class binop_oneuse<SDPatternOperator operator>
> >     : PatFrag<(ops node:$A, node:$B),
> >               (operator node:$A, node:$B), [{
> >   return N->hasOneUse();
> > }]>;`
> > 
> > def add_su : binop_oneuse<add>;
> > def and_su : binop_oneuse<and>;
> > def srl_su : binop_oneuse<srl>;
> > 
> > class unop_oneuse<SDPatternOperator operator>
> >     : PatFrag<(ops node:$A),
> >               (operator node:$A), [{
> >   return N->hasOneUse();
> > }]>;
> > 
> > def ineg_su : unop_oneuse<ineg>;
> > def trunc_su : unop_oneuse<trunc>;
> > ```
> 
> Makes sense, will do for V2.
> 
> Any thoughts on:
> """
> Note, this may cause more mov instructions to be emitted sometimes
> because BMI instructions only have 1 src and write-only to dst. A
> better approach may be to only avoid BMI for (and/xor X, (add/sub
> 0/-1, X)) if this is the last use of X but NOT the last use of
> (add/sub 0/-1, X).
> """
> ?
Microarchitecture usually can eliminate mov instructions by register renaming, so we can consider no performance lost except for code size.


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