[llvm] [RISCV] Avoid matching 3/5/9 * 2^N as 2^N + 2/4/8 (e.g. 24) (PR #88937)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Philip Reames (preames)

<details>
<summary>Changes</summary>

The former is better as a zero extend can be folded into the sll, whereas the later currently produces a seperate zext.w due to bad interactions with other combines.

---
Full diff: https://github.com/llvm/llvm-project/pull/88937.diff


3 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+6) 
- (modified) llvm/test/CodeGen/RISCV/addimm-mulimm.ll (+4-5) 
- (modified) llvm/test/CodeGen/RISCV/rv64zba.ll (+22) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 765838aafb58d2..de2ad639f0d6c8 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -13416,6 +13416,12 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
     return SDValue();
   uint64_t MulAmt = CNode->getZExtValue();
 
+  // 3/5/9 * 2^N -> shXadd (sll X, C), (sll X, C)
+  // Matched in tablegen, avoid perturbing patterns.
+  for (uint64_t Divisor : {3, 5, 9})
+    if (MulAmt % Divisor == 0 && isPowerOf2_64(MulAmt / Divisor))
+      return SDValue();
+
   // If this is a power 2 + 2/4/8, we can use a shift followed by a single
   // shXadd. First check if this a sum of two power of 2s because that's
   // easy. Then count how many zeros are up to the first bit.
diff --git a/llvm/test/CodeGen/RISCV/addimm-mulimm.ll b/llvm/test/CodeGen/RISCV/addimm-mulimm.ll
index 10103f071462c5..48fa69e1045656 100644
--- a/llvm/test/CodeGen/RISCV/addimm-mulimm.ll
+++ b/llvm/test/CodeGen/RISCV/addimm-mulimm.ll
@@ -551,9 +551,8 @@ define i64 @add_mul_combine_infinite_loop(i64 %x) {
 ; RV32IMB-NEXT:    sh3add a1, a1, a2
 ; RV32IMB-NEXT:    sh1add a0, a0, a0
 ; RV32IMB-NEXT:    slli a2, a0, 3
-; RV32IMB-NEXT:    li a3, 1
-; RV32IMB-NEXT:    slli a3, a3, 11
-; RV32IMB-NEXT:    sh3add a0, a0, a3
+; RV32IMB-NEXT:    addi a0, a2, 2047
+; RV32IMB-NEXT:    addi a0, a0, 1
 ; RV32IMB-NEXT:    sltu a2, a0, a2
 ; RV32IMB-NEXT:    add a1, a1, a2
 ; RV32IMB-NEXT:    ret
@@ -562,8 +561,8 @@ define i64 @add_mul_combine_infinite_loop(i64 %x) {
 ; RV64IMB:       # %bb.0:
 ; RV64IMB-NEXT:    addi a0, a0, 86
 ; RV64IMB-NEXT:    sh1add a0, a0, a0
-; RV64IMB-NEXT:    slli a0, a0, 3
-; RV64IMB-NEXT:    addi a0, a0, -16
+; RV64IMB-NEXT:    li a1, -16
+; RV64IMB-NEXT:    sh3add a0, a0, a1
 ; RV64IMB-NEXT:    ret
   %tmp0 = mul i64 %x, 24
   %tmp1 = add i64 %tmp0, 2048
diff --git a/llvm/test/CodeGen/RISCV/rv64zba.ll b/llvm/test/CodeGen/RISCV/rv64zba.ll
index a84b9e5e7962f6..c3c757656be933 100644
--- a/llvm/test/CodeGen/RISCV/rv64zba.ll
+++ b/llvm/test/CodeGen/RISCV/rv64zba.ll
@@ -2490,3 +2490,25 @@ define ptr @test_gep_gep_dont_crash(ptr %p, i64 %a1, i64 %a2) {
   %gep2 = getelementptr i64, ptr %gep1, i64 %a1
   ret ptr %gep2
 }
+
+define i64 @regression(i32 signext %x, i32 signext %y) {
+; RV64I-LABEL: regression:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    subw a0, a0, a1
+; RV64I-NEXT:    slli a0, a0, 32
+; RV64I-NEXT:    li a1, 3
+; RV64I-NEXT:    slli a1, a1, 35
+; RV64I-NEXT:    mulhu a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBA-LABEL: regression:
+; RV64ZBA:       # %bb.0:
+; RV64ZBA-NEXT:    subw a0, a0, a1
+; RV64ZBA-NEXT:    slli.uw a0, a0, 3
+; RV64ZBA-NEXT:    sh1add a0, a0, a0
+; RV64ZBA-NEXT:    ret
+  %sub = sub i32 %x, %y
+  %ext = zext i32 %sub to i64
+  %res = mul nuw nsw i64 %ext, 24
+  ret i64 %res
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/88937