[llvm] Add FeatureFuseLiterals as SubTargetFeature for Grace and Olympus (PR #160257)

[Sushant Gokhale via llvm-commits]

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@@ -12564,6 +12564,17 @@ bool AArch64TargetLowering::isOffsetFoldingLegal(
 
 bool AArch64TargetLowering::isFPImmLegal(const APFloat &Imm, EVT VT,
                                          bool OptForSize) const {
+  // If the constant to be materialized is scalar, it maybe efficient to use
+  // sequence of 'mov + fmov' rather than 'adrp + ldr' on specified CPU's.
+  // However, when materializing vector of constants, there are two things to
+  // note:
+  // 1. Throughput of fmov instruction is very low.
+  // 2. ldr instruction can load multiple constants in one go. Also, it's
+  // throughput is higher as compared to fmov.
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sushgokh wrote:

> "fmovs limit throughput, loads are great". 

We want to be cautious when we are materializing vector of constants. So, I have used "maybe more efficient" to describe that we are pessimistic here.

https://github.com/llvm/llvm-project/pull/160257