[LoongArch] Override cost hooks to expose more DAG combine opportunities

[zhaoqi5]

No description provided.

[llvmbot]

@llvm/pr-subscribers-backend-loongarch

Author: ZhaoQi (zhaoqi5)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/157824.diff

2 Files Affected:

• (modified) llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp (+17)
• (modified) llvm/lib/Target/LoongArch/LoongArchISelLowering.h (+4)

diff --git a/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp b/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp
index 634914d3b3fd0..f3733f04d13d3 100644
--- a/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp
+++ b/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp
@@ -9094,3 +9094,20 @@ bool LoongArchTargetLowering::SimplifyDemandedBitsForTargetNode(
   return TargetLowering::SimplifyDemandedBitsForTargetNode(
       Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
 }
+
+bool LoongArchTargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
+                                                      unsigned Index) const {
+  if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
+    return false;
+
+  // Extract a 128-bit subvector from index 0 of a 256-bit vector is free.
+  return Index == 0;
+}
+
+bool LoongArchTargetLowering::isExtractVecEltCheap(EVT VT,
+                                                   unsigned Index) const {
+  EVT EltVT = VT.getScalarType();
+
+  // Extract a scalar FP value from index 0 of a vector is free.
+  return (EltVT == MVT::f32 || EltVT == MVT::f64) && Index == 0;
+}
diff --git a/llvm/lib/Target/LoongArch/LoongArchISelLowering.h b/llvm/lib/Target/LoongArch/LoongArchISelLowering.h
index 9d14934a9d363..1495e98829e74 100644
--- a/llvm/lib/Target/LoongArch/LoongArchISelLowering.h
+++ b/llvm/lib/Target/LoongArch/LoongArchISelLowering.h
@@ -332,6 +332,10 @@ class LoongArchTargetLowering : public TargetLowering {
                                          TargetLoweringOpt &TLO,
                                          unsigned Depth) const override;
 
+  bool isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
+                               unsigned Index) const override;
+  bool isExtractVecEltCheap(EVT VT, unsigned Index) const override;
+
 private:
   /// Target-specific function used to lower LoongArch calling conventions.
   typedef bool LoongArchCCAssignFn(const DataLayout &DL, LoongArchABI::ABI ABI,

[SixWeining]

Are there any test cases that have benefited from this?

[zhaoqi5]

Are there any test cases that have benefited from this?

Mark extract floating point element from index 0 as cheap will lead to scalarize. Currently, we use float loads to get the scalar floating point constant, but use code sequence to generate the constants in vectors. So scalarization will get worse code in some cases.

A new pr (#158050) was just pushed to change the way of getting fp constant. After it is approved, I will find some tests for this commit.

[heiher]

LGTM.