[llvm] [RISCV] Account for ADDI immediate range in select of two constants w/ zicond (PR #155471)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Philip Reames (preames)

<details>
<summary>Changes</summary>

When choosing to materialize a select of two constants using zicond, we have a choice of which direction to compute the delta.  The prior cost was looking only at the cost of the values without accounting for the fact it's actually the delta which is the highest cost and that sometimes the addend can fold into an addi.

Note that the addi is also preferable since it might be folded into a dependent memory instruction if e.g. we're doing a load from a select of two constant addresses.

---
Full diff: https://github.com/llvm/llvm-project/pull/155471.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+15-5) 
- (modified) llvm/test/CodeGen/RISCV/select-const.ll (+6-8) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index b84bd1ce0ac50..beee74f4d9215 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -9280,11 +9280,21 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
         }
       }
 
-      const int TrueValCost = RISCVMatInt::getIntMatCost(
-          TrueVal, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
-      const int FalseValCost = RISCVMatInt::getIntMatCost(
-          FalseVal, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
-      bool IsCZERO_NEZ = TrueValCost <= FalseValCost;
+      auto getCost = [&](APInt Delta, APInt Addend) {
+        const int DeltaCost = RISCVMatInt::getIntMatCost(
+            Delta, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
+        // Dos the addend folds into an ADDI
+        if (Addend.isSignedIntN(12))
+          return DeltaCost;
+        const int AddendCost = RISCVMatInt::getIntMatCost(
+            Addend, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
+        // Panalize the ADD slightly so that we prefer to end with an ADDI
+        // if costs are otherwise equal.  This helps to expose the immediate
+        // for possible folding into a dependent memory instruction.
+        return AddendCost + DeltaCost + 1;
+      };
+      bool IsCZERO_NEZ = getCost(FalseVal - TrueVal, TrueVal) <=
+                         getCost(TrueVal - FalseVal, FalseVal);
       SDValue LHSVal = DAG.getConstant(
           IsCZERO_NEZ ? FalseVal - TrueVal : TrueVal - FalseVal, DL, VT);
       SDValue RHSVal =
diff --git a/llvm/test/CodeGen/RISCV/select-const.ll b/llvm/test/CodeGen/RISCV/select-const.ll
index c11b5d9b4a71d..e8019e99a6da9 100644
--- a/llvm/test/CodeGen/RISCV/select-const.ll
+++ b/llvm/test/CodeGen/RISCV/select-const.ll
@@ -506,11 +506,10 @@ define i32 @select_nonnegative_lui_addi(i32 signext %x) {
 ; RV32ZICOND-LABEL: select_nonnegative_lui_addi:
 ; RV32ZICOND:       # %bb.0:
 ; RV32ZICOND-NEXT:    srli a0, a0, 31
-; RV32ZICOND-NEXT:    lui a1, 1048572
-; RV32ZICOND-NEXT:    addi a1, a1, 25
-; RV32ZICOND-NEXT:    czero.eqz a0, a1, a0
 ; RV32ZICOND-NEXT:    lui a1, 4
-; RV32ZICOND-NEXT:    add a0, a0, a1
+; RV32ZICOND-NEXT:    addi a1, a1, -25
+; RV32ZICOND-NEXT:    czero.nez a0, a1, a0
+; RV32ZICOND-NEXT:    addi a0, a0, 25
 ; RV32ZICOND-NEXT:    ret
 ;
 ; RV64I-LABEL: select_nonnegative_lui_addi:
@@ -536,11 +535,10 @@ define i32 @select_nonnegative_lui_addi(i32 signext %x) {
 ; RV64ZICOND-LABEL: select_nonnegative_lui_addi:
 ; RV64ZICOND:       # %bb.0:
 ; RV64ZICOND-NEXT:    srli a0, a0, 63
-; RV64ZICOND-NEXT:    lui a1, 1048572
-; RV64ZICOND-NEXT:    addi a1, a1, 25
-; RV64ZICOND-NEXT:    czero.eqz a0, a1, a0
 ; RV64ZICOND-NEXT:    lui a1, 4
-; RV64ZICOND-NEXT:    add a0, a0, a1
+; RV64ZICOND-NEXT:    addi a1, a1, -25
+; RV64ZICOND-NEXT:    czero.nez a0, a1, a0
+; RV64ZICOND-NEXT:    addi a0, a0, 25
 ; RV64ZICOND-NEXT:    ret
   %cmp = icmp sgt i32 %x, -1
   %cond = select i1 %cmp, i32 16384, i32 25

``````````

</details>


https://github.com/llvm/llvm-project/pull/155471