[RISCV] Improve codegen for i32 udiv/urem by constant on RV64.

[craig.topper]

Summary

The division by constant optimization often produces constants that
are uimm32, but not simm32. These constants require 3 or 4 instructions
to materialize without Zba.

Since these instructions are often used by a multiply with a LHS
that needs to be zero extended with an AND, we can switch the MUL
to a MULHU by shifting both inputs left by 32. Once we shift the
constant left, the upper 32 bits no longer need to be 0 so constant
materialization is free to use LUI+ADDIW. This reduces the constant
materialization from 4 instructions to 3 in some cases while also
reducing the zero extend of the LHS from 2 shifts to 1.

[mcberg2021]

llvm/lib/Target/RISCV/RISCVInstrInfoM.td

112

Looks like the TODO is done making that part of the comment no longer needed

[craig.topper]

llvm/lib/Target/RISCV/RISCVInstrInfoM.td

112

I think when I wrote the TODO I meant to add something like these patterns

(i64 (mul (and GPR:$rs1, 0xffffffff), (assertzexti32 GPR:$rs2)))

and

(i64 (mul (assertzexti32 GPR:$rs1), (and GPR:$rs2, 0xffffffff)))

Where assertzexti32 covers AssertZExt and anything that computeKnownBits can prove is has only 32 active bits. In that case we'd insert a SLLI on the assertexti32 side without reducing any instructions for that operand. If that operand is the critical path it would make it worse. If the AND has an additional use it would be an increase in code size.

SelectionDAG doesn't normally check the critical path like that, so maybe I should drop the TODO based on the fact that we couldn't check that.

[craig.topper]

Remove TODO

[mcberg2021]

LGTM

llvm/lib/Target/RISCV/RISCVInstrInfoM.td

 
 // Although the sexti32 operands may not have originated from an i32 srem,
 // this pattern is safe as it is impossible for two sign extended inputs to
 // produce a result where res[63:32]=0 and res[31]=1.
 def : Pat<(srem (sexti32 (i64 GPR:$rs1)), (sexti32 (i64 GPR:$rs2))),
           (REMW GPR:$rs1, GPR:$rs2)>;
 } // Predicates = [HasStdExtM, IsRV64]
 
+// Pattern to detect constants with no more than 32 active bits that can't
+// be materialized with lui+addiw.
+def uimm32_not_simm32 : PatLeaf<(XLenVT GPR:$a), [{
+  auto *C = dyn_cast<ConstantSDNode>(N);
+  return C && C->hasOneUse() && isUInt<32>(C->getZExtValue()) &&
+         !isInt<32>(C->getSExtValue());
+}]>;
+
 let Predicates = [HasStdExtM, IsRV64, NotHasStdExtZba] in {
 // Special case for calculating the full 64-bit product of a 32x32 unsigned
 // multiply where the inputs aren't known to be zero extended. We can shift the
 // inputs left by 32 and use a MULHU. This saves two SRLIs needed to finish
 // zeroing the upper 32 bits.
 // TODO: If one of the operands is zero extended and the other isn't, we might
 // still be better off shifting both left by 32.
 def : Pat<(i64 (mul (and GPR:$rs1, 0xffffffff), (and GPR:$rs2, 0xffffffff))),
           (MULHU (SLLI GPR:$rs1, 32), (SLLI GPR:$rs2, 32))>;
+// The RHS could also be a constant that is hard to materialize. By shifting
+// left we can allow constant materialization to use LUI+ADDIW via
+// hasAllWUsers.
+def : Pat<(i64 (mul (and GPR:$rs1, 0xffffffff), uimm32_not_simm32:$rs2)),
+          (MULHU (SLLI GPR:$rs1, 32), (SLLI GPR:$rs2, 32))>;
 } // Predicates = [HasStdExtM, IsRV64, NotHasStdExtZba]

llvm/test/CodeGen/RISCV/div.ll

 ; RV64I-NEXT:    call __udivdi3@plt
 ; RV64I-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
 ; RV64I-NEXT:    addi sp, sp, 16
 ; RV64I-NEXT:    ret
 ;
 ; RV64IM-LABEL: udiv_constant:
 ; RV64IM:       # %bb.0:
 ; RV64IM-NEXT:    slli a0, a0, 32
-; RV64IM-NEXT:    srli a0, a0, 32
-; RV64IM-NEXT:    lui a1, 205
+; RV64IM-NEXT:    lui a1, 838861
 ; RV64IM-NEXT:    addiw a1, a1, -819
-; RV64IM-NEXT:    slli a1, a1, 12
-; RV64IM-NEXT:    addi a1, a1, -819
-; RV64IM-NEXT:    mul a0, a0, a1
+; RV64IM-NEXT:    slli a1, a1, 32
+; RV64IM-NEXT:    mulhu a0, a0, a1
 ; RV64IM-NEXT:    srli a0, a0, 34
 ; RV64IM-NEXT:    ret
   %1 = udiv i32 %a, 5
   ret i32 %1
 }
 
 define i32 @udiv_pow2(i32 %a) nounwind {
 ; RV32I-LABEL: udiv_pow2:

llvm/test/CodeGen/RISCV/urem-lkk.ll

 ; RV64I-NEXT:    call __umoddi3@plt
 ; RV64I-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
 ; RV64I-NEXT:    addi sp, sp, 16
 ; RV64I-NEXT:    ret
 ;
 ; RV64IM-LABEL: fold_urem_positive_even:
 ; RV64IM:       # %bb.0:
 ; RV64IM-NEXT:    slli a1, a0, 32
-; RV64IM-NEXT:    srli a1, a1, 32
-; RV64IM-NEXT:    lui a2, 253241
-; RV64IM-NEXT:    slli a2, a2, 2
-; RV64IM-NEXT:    addi a2, a2, -61
-; RV64IM-NEXT:    mul a1, a1, a2
+; RV64IM-NEXT:    lui a2, 1012964
+; RV64IM-NEXT:    addiw a2, a2, -61
+; RV64IM-NEXT:    slli a2, a2, 32
+; RV64IM-NEXT:    mulhu a1, a1, a2
 ; RV64IM-NEXT:    srli a1, a1, 42
 ; RV64IM-NEXT:    addi a2, zero, 1060
 ; RV64IM-NEXT:    mulw a1, a1, a2
 ; RV64IM-NEXT:    subw a0, a0, a1
 ; RV64IM-NEXT:    ret
   %1 = urem i32 %x, 1060
   ret i32 %1
 }