[llvm] [AMDGPU] Implement LSR cost model for GFX9+ (PR #184138)

[Matt Arsenault via llvm-commits]

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@@ -1703,3 +1704,51 @@ GCNTTIImpl::getInstructionUniformity(const Value *V) const {
 
   return InstructionUniformity::Default;
 }
+
+InstructionCost GCNTTIImpl::getScalingFactorCost(Type *Ty, GlobalValue *BaseGV,
+                                                 StackOffset BaseOffset,
+                                                 bool HasBaseReg, int64_t Scale,
+                                                 unsigned AddrSpace) const {
+  if (HasBaseReg && Scale != 0) {
+    // gfx1250+ can fold base+scale*index into the instruction when scale
+    // equals the memory access size (scale_offset bit). This is supported
+    // for global/constant/flat/scratch but NOT for LDS or GDS.
+    // GDS does not exist on gfx1250+, but we exclude REGION_ADDRESS for
+    // correctness since the address space is still representable in IR.
+    if (getST()->hasScaleOffset() && Ty && Ty->isSized() &&
+        AddrSpace != AMDGPUAS::LOCAL_ADDRESS &&
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arsenm wrote:

We do use constant address space as a weak heuristic that it will use SMEM instructions 

https://github.com/llvm/llvm-project/pull/184138