[llvm] [CostModel][AArch64] Make extractelement, with fmul user, free whenev… (PR #111479)

[Sushant Gokhale via llvm-commits]

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@@ -3194,6 +3206,149 @@ InstructionCost AArch64TTIImpl::getVectorInstrCostHelper(const Instruction *I,
     // compile-time considerations.
   }
 
+  // In case of Neon, if there exists extractelement from lane != 0 such that
+  // 1. extractelement does not necessitate a move from vector_reg -> GPR.
+  // 2. extractelement result feeds into fmul.
+  // 3. Other operand of fmul is a scalar or extractelement from lane 0 or lane
+  // equivalent to 0.
+  // then the extractelement can be merged with fmul in the backend and it
+  // incurs no cost.
+  // e.g.
+  // define double @foo(<2 x double> %a) {
+  //   %1 = extractelement <2 x double> %a, i32 0
+  //   %2 = extractelement <2 x double> %a, i32 1
+  //   %res = fmul double %1, %2
+  //   ret double %res
+  // }
+  // %2 and %res can be merged in the backend to generate fmul v0, v0, v1.d[1]
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sushgokh wrote:

by mistake. Will correct this.

https://github.com/llvm/llvm-project/pull/111479