[PATCH] D149169: [RISCV] Cost constant materialization of vectors in phis

[Alexey Bataev via Phabricator via llvm-commits]

ABataev added inline comments.


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Comment at: llvm/test/Analysis/CostModel/RISCV/rvv-phi-const.ll:8
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: br label %d
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %x = phi <2 x i8> [ <i8 1, i8 -1>, %a ], [ <i8 -1, i8 1>, %b ]
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %x = phi <2 x i8> [ <i8 1, i8 -1>, %a ], [ <i8 -1, i8 1>, %b ]
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: ret <2 x i8> %x
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luke wrote:
> ABataev wrote:
> > luke wrote:
> > > The generated code:
> > > ```
> > > f:                                      # @f
> > > 	.cfi_startproc
> > > # %bb.0:
> > > 	vsetivli	zero, 2, e8, mf8, ta, ma
> > > 	vid.v	v8
> > > 	andi	a0, a0, 1
> > > 	vadd.vv	v8, v8, v8
> > > 	beqz	a0, .LBB0_2
> > > # %bb.1:
> > > 	vrsub.vi	v8, v8, 1
> > > 	ret
> > > .LBB0_2:                                # %b
> > > 	vadd.vi	v8, v8, -1
> > > 	ret
> > > ```
> > I have one concern here about loops. In the loop we need to account for the materialization cost only once, if the input constant is a live-in. So for such values better to consider it free, I assume.
> That's a good point. The cost of constants in stores must also be currently affected by this too right?
Looks so. But it requires some significant rework of the cost model, I assume. For phi's you can try just to check if there is a loop in the graph (with the small enough limit), without actual analysis/use of the Loop objects.


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  https://reviews.llvm.org/D149169/new/

https://reviews.llvm.org/D149169