[RISCV] Refine the (mul (zext.w X), C) -> mulhu isel heuristic.

We try to shift both X and C left by 32 to replace the zext.w
with a SLLI and use mulhu. If C is already a simm32, this likely
makes a constant that is more expensive to materialize.

2 files changed, 37 insertions, 20 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
index 2824472..994aa94 100644
--- a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -1065,17 +1065,21 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
     if (!isMask_64(C2))
       break;
 
-    // If this can be an ANDI, ZEXT.H or ZEXT.W, don't do this if the ANDI/ZEXT
-    // has multiple users or the constant is a simm12. This prevents inserting
-    // a shift and still have uses of the AND/ZEXT. Shifting a simm12 will
-    // likely make it more costly to materialize. Otherwise, using a SLLI
-    // might allow it to be compressed.
+    // If this can be an ANDI or ZEXT.H, don't do this if the ANDI/ZEXT has
+    // multiple users or the constant is a simm12. This prevents inserting a
+    // shift and still have uses of the AND/ZEXT. Shifting a simm12 will likely
+    // make it more costly to materialize. Otherwise, using a SLLI might allow
+    // it to be compressed.
     bool IsANDIOrZExt =
         isInt<12>(C2) ||
-        (C2 == UINT64_C(0xFFFF) && Subtarget->hasStdExtZbb()) ||
-        (C2 == UINT64_C(0xFFFFFFFF) && Subtarget->hasStdExtZba());
+        (C2 == UINT64_C(0xFFFF) && Subtarget->hasStdExtZbb());
     if (IsANDIOrZExt && (isInt<12>(N1C->getSExtValue()) || !N0.hasOneUse()))
       break;
+    // If this can be a ZEXT.w, don't do this if the ZEXT has multiple users or
+    // the constant is a simm32.
+    bool IsZExtW = C2 == UINT64_C(0xFFFFFFFF) && Subtarget->hasStdExtZba();
+    if (IsZExtW && (isInt<32>(N1C->getSExtValue()) || !N0.hasOneUse()))
+      break;
 
     // We need to shift left the AND input and C1 by a total of XLen bits.
 
diff --git a/llvm/test/CodeGen/RISCV/div-by-constant.ll b/llvm/test/CodeGen/RISCV/div-by-constant.ll
index da30456..05200f3 100644
--- a/llvm/test/CodeGen/RISCV/div-by-constant.ll
+++ b/llvm/test/CodeGen/RISCV/div-by-constant.ll
@@ -46,19 +46,32 @@ define i32 @udiv_constant_add(i32 %a) nounwind {
 ; RV32-NEXT:    srli a0, a0, 2
 ; RV32-NEXT:    ret
 ;
-; RV64-LABEL: udiv_constant_add:
-; RV64:       # %bb.0:
-; RV64-NEXT:    slli a1, a0, 32
-; RV64-NEXT:    lui a2, 149797
-; RV64-NEXT:    addiw a2, a2, -1755
-; RV64-NEXT:    slli a2, a2, 32
-; RV64-NEXT:    mulhu a1, a1, a2
-; RV64-NEXT:    srli a1, a1, 32
-; RV64-NEXT:    subw a0, a0, a1
-; RV64-NEXT:    srliw a0, a0, 1
-; RV64-NEXT:    add a0, a0, a1
-; RV64-NEXT:    srli a0, a0, 2
-; RV64-NEXT:    ret
+; RV64IM-LABEL: udiv_constant_add:
+; RV64IM:       # %bb.0:
+; RV64IM-NEXT:    slli a1, a0, 32
+; RV64IM-NEXT:    lui a2, 149797
+; RV64IM-NEXT:    addiw a2, a2, -1755
+; RV64IM-NEXT:    slli a2, a2, 32
+; RV64IM-NEXT:    mulhu a1, a1, a2
+; RV64IM-NEXT:    srli a1, a1, 32
+; RV64IM-NEXT:    subw a0, a0, a1
+; RV64IM-NEXT:    srliw a0, a0, 1
+; RV64IM-NEXT:    add a0, a0, a1
+; RV64IM-NEXT:    srli a0, a0, 2
+; RV64IM-NEXT:    ret
+;
+; RV64IMZB-LABEL: udiv_constant_add:
+; RV64IMZB:       # %bb.0:
+; RV64IMZB-NEXT:    zext.w a1, a0
+; RV64IMZB-NEXT:    lui a2, 149797
+; RV64IMZB-NEXT:    addiw a2, a2, -1755
+; RV64IMZB-NEXT:    mul a1, a1, a2
+; RV64IMZB-NEXT:    srli a1, a1, 32
+; RV64IMZB-NEXT:    subw a0, a0, a1
+; RV64IMZB-NEXT:    srliw a0, a0, 1
+; RV64IMZB-NEXT:    add a0, a0, a1
+; RV64IMZB-NEXT:    srli a0, a0, 2
+; RV64IMZB-NEXT:    ret
   %1 = udiv i32 %a, 7
   ret i32 %1
 }