[PATCH] D132322: [AArch64][SelectionDAG] Optimize multiplication by constant

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: llvm/test/CodeGen/AArch64/mul_pow2.ll:306
+; GISEL-NEXT:    sub w0, w8, #1
+; GISEL-NEXT:    ret
+  %mul = mul nsw i32 %x, 6
----------------
Allen wrote:
> efriedma wrote:
> > There is one other possible sequence here:
> > 
> > ```
> > mov w8, #6
> > mov w9, #-1
> > madd w0, w0, w8, w9
> > ```
> > 
> > This is obviously not that exciting in isolation, but it might make sense if we can hoist the "mov" instructions out of a loop.
> hi @efriedma, I try to generate the new sequence as you showed, but I don't know how to put a const into a register? as the operand of madd should not be const value.
> 
>   For the const **AddSub->getOperand(1)**, even when I use a ISD::ADD , it still retrun a const value.
> ```
>      SDValue Const = DAG.getNode(ISD::ADD, DL, VT, AddSub->getOperand(1), DAG.getConstant(0, DL, VT));
> 
>     (gdb) p Const->dump()
>     t5: i32 = Constant<-1>
>     $10 = void
>     (gdb) p AddSub->getOperand(1)->dump()
>      t5: i32 = Constant<-1>
> ```
You don't have to do anything special to "put a const in a register".  For example, if you write `unsigned a(unsigned x) { return x * 0x33333333 + 0xF0F0F0F0; }`, you get code like I suggested.  Getting the same code with smaller immediates is just a matter of making sure we pick the pattern for madd, and not the pattern for sub-with-imm.


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  https://reviews.llvm.org/D132322/new/

https://reviews.llvm.org/D132322