[llvm] [CostModel][AArch64][ARM][AMDGPU] Add generic udiv/urem by power-2 costs (PR #126912)

[David Green via llvm-commits]

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@@ -990,6 +990,17 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
       return LT.first * 2 * OpCost;
     }
 
+    // For a UDIV/UREM, if the operand is a power of 2 we can use the shift or
+    // and cost.
+    if (ISD == ISD::UDIV && Opd2Info.isPowerOf2())
+      return thisT()->getArithmeticInstrCost(Instruction::LShr, Ty, CostKind,
+                                             Opd1Info.getNoProps(),
+                                             Opd2Info.getNoProps());
+    if (ISD == ISD::UREM && Opd2Info.isPowerOf2())
+      return thisT()->getArithmeticInstrCost(Instruction::And, Ty, CostKind,
+                                             Opd1Info.getNoProps(),
+                                             Opd2Info.getNoProps());
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davemgreen wrote:

It can come up from fshl costs and anything else that is checking the cost of a div/rem by a constant amount, that might or might not be power-2. i.e. hypothetical instructions, not real ones.

I could change the funnel shift cost directly and add this to aarch64/arm as needed. X86 already has this code, so gets better funnel shift costs. I get that it would cost a little bit of compile time for something that rarely comes up.

https://github.com/llvm/llvm-project/pull/126912