[PATCH] D132858: [AArch64][TTI] Set the cost of XTN to 1 for 2xi64 (to 2xi32) and 8xi16 (to8xi8).

Mon Aug 29 08:49:47 PDT 2022

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>From [1], XTN is used for {v8i16->v8i8, v4i32->v4i16, v2i64->v2i32}.
Without this, an IR instruction like trunc <8 x i16> %v to <8 x i8> is considered free and might be sinked to other basic blocks. As a result, the sinked 'trunc' is in a different basic block with its (usually not-free) vector operand and misses the chance to be combined during instruction selection. (examples in [2])

[1] https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L4472
[2] examples

- trunc(umin(X, 255)) -> UQXTRN v8i8 (and other {u,s}x{min,max} pattern for v8i16 operands) from https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L4515-L4528
- trunc (AArch64vlshr v8i16, imm) -> SHRNv8i8 (same missed for SHRNv2i32) from https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L6743-L6748


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132858

Files:
  llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
  llvm/test/Analysis/CostModel/AArch64/cast.ll


Index: llvm/test/Analysis/CostModel/AArch64/cast.ll
===================================================================

--- llvm/test/Analysis/CostModel/AArch64/cast.ll
+++ llvm/test/Analysis/CostModel/AArch64/cast.ll
@@ -166,20 +166,20 @@
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s2i8i64 = trunc <2 x i64> undef to <2 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i16i32 = trunc <2 x i32> undef to <2 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i16i64 = trunc <2 x i64> undef to <2 x i16>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i32i64 = trunc <2 x i64> undef to <2 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s2i32i64 = trunc <2 x i64> undef to <2 x i32>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i16 = trunc <4 x i16> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i32 = trunc <4 x i32> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i64 = trunc <4 x i64> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s4i16i32 = trunc <4 x i32> undef to <4 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i16i64 = trunc <4 x i64> undef to <4 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i32i64 = trunc <4 x i64> undef to <4 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i8i16 = trunc <8 x i16> undef to <8 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s8i8i16 = trunc <8 x i16> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s8i8i32 = trunc <8 x i32> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i8i64 = trunc <8 x i64> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s8i16i32 = trunc <8 x i32> undef to <8 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s8i16i64 = trunc <8 x i64> undef to <8 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i32i64 = trunc <8 x i64> undef to <8 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s16i8i16 = trunc <16 x i16> undef to <16 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s16i8i16 = trunc <16 x i16> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %s16i8i32 = trunc <16 x i32> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s16i8i64 = trunc <16 x i64> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %s16i16i32 = trunc <16 x i32> undef to <16 x i16>
Index: llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -1602,7 +1602,9 @@
 
   static const TypeConversionCostTblEntry
   ConversionTbl[] = {
-    { ISD::TRUNCATE, MVT::v4i16, MVT::v4i32,  1 },
+    { ISD::TRUNCATE, MVT::v4i16, MVT::v4i32,  1 },  // xtn
+    { ISD::TRUNCATE, MVT::v8i8,  MVT::v8i16,  1 },  // xtn
+    { ISD::TRUNCATE, MVT::v2i32, MVT::v2i64,  1 },  // xtn
     { ISD::TRUNCATE, MVT::v4i32, MVT::v4i64,  0 },
     { ISD::TRUNCATE, MVT::v8i8,  MVT::v8i32,  3 },
     { ISD::TRUNCATE, MVT::v16i8, MVT::v16i32, 6 },


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