[RISCV] Use short forward branch for ISD::ABS.

We can use short forward branch to conditionally negate if the
value is negative.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -396,9 +396,14 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
       setOperationAction(ISD::CTLZ, MVT::i32, Expand);
   }
 
-  if (!RV64LegalI32 && Subtarget.is64Bit())
+  if (!RV64LegalI32 && Subtarget.is64Bit() &&
+      !Subtarget.hasShortForwardBranchOpt())
     setOperationAction(ISD::ABS, MVT::i32, Custom);
 
+  // We can use PseudoCCSUB to implement ABS.
+  if (Subtarget.hasShortForwardBranchOpt())
+    setOperationAction(ISD::ABS, XLenVT, Legal);
+
   if (!Subtarget.hasVendorXTHeadCondMov())
     setOperationAction(ISD::SELECT, XLenVT, Custom);
 

llvm/lib/Target/RISCV/RISCVInstrInfo.td

@@ -1242,6 +1242,10 @@ def anyext_oneuse : unop_oneuse<anyext>;
 def ext_oneuse    : unop_oneuse<ext>;
 def fpext_oneuse  : unop_oneuse<any_fpextend>;
 
+def 33signbits_node : PatLeaf<(i64 GPR:$src), [{
+  return CurDAG->ComputeNumSignBits(SDValue(N, 0)) > 32;
+}]>;
+
 /// Simple arithmetic operations
 
 def : PatGprGpr<add, ADD>;
@@ -2002,6 +2006,15 @@ def : Pat<(binop_allwusers<add> GPR:$rs1, (AddiPair:$rs2)),
                  (AddiPairImmSmall AddiPair:$rs2))>;
 }
 
+let Predicates = [HasShortForwardBranchOpt] in
+def : Pat<(XLenVT (abs GPR:$rs1)),
+          (PseudoCCSUB (XLenVT GPR:$rs1), (XLenVT X0), /* COND_LT */ 2,
+           (XLenVT GPR:$rs1), (XLenVT X0), (XLenVT GPR:$rs1))>;
+let Predicates = [HasShortForwardBranchOpt, IsRV64] in
+def : Pat<(sext_inreg (abs 33signbits_node:$rs1), i32),
+          (PseudoCCSUBW (i64 GPR:$rs1), (i64 X0), /* COND_LT */ 2,
+           (XLenVT GPR:$rs1), (i64 X0), (i64 GPR:$rs1))>;
+
 //===----------------------------------------------------------------------===//
 // Experimental RV64 i32 legalization patterns.
 //===----------------------------------------------------------------------===//

llvm/test/CodeGen/RISCV/short-forward-branch-opt.ll

@@ -1458,3 +1458,81 @@ entry:
  %1 = select i1 %cond, i32 %C, i32 %0
  ret i32 %1
 }
+
+define signext i32 @abs_i32(i32 signext %x) {
+; NOSFB-LABEL: abs_i32:
+; NOSFB:       # %bb.0:
+; NOSFB-NEXT:    sraiw a1, a0, 31
+; NOSFB-NEXT:    xor a0, a0, a1
+; NOSFB-NEXT:    subw a0, a0, a1
+; NOSFB-NEXT:    ret
+;
+; RV64SFB-LABEL: abs_i32:
+; RV64SFB:       # %bb.0:
+; RV64SFB-NEXT:    bgez a0, .LBB34_2
+; RV64SFB-NEXT:  # %bb.1:
+; RV64SFB-NEXT:    negw a0, a0
+; RV64SFB-NEXT:  .LBB34_2:
+; RV64SFB-NEXT:    ret
+;
+; ZICOND-LABEL: abs_i32:
+; ZICOND:       # %bb.0:
+; ZICOND-NEXT:    bgez a0, .LBB34_2
+; ZICOND-NEXT:  # %bb.1:
+; ZICOND-NEXT:    negw a0, a0
+; ZICOND-NEXT:  .LBB34_2:
+; ZICOND-NEXT:    ret
+;
+; RV32SFB-LABEL: abs_i32:
+; RV32SFB:       # %bb.0:
+; RV32SFB-NEXT:    bgez a0, .LBB34_2
+; RV32SFB-NEXT:  # %bb.1:
+; RV32SFB-NEXT:    neg a0, a0
+; RV32SFB-NEXT:  .LBB34_2:
+; RV32SFB-NEXT:    ret
+  %a = call i32 @llvm.abs.i32(i32 %x, i1 false)
+  ret i32 %a
+}
+declare i32 @llvm.abs.i32(i32, i1)
+
+define i64 @abs_i64(i64 %x) {
+; NOSFB-LABEL: abs_i64:
+; NOSFB:       # %bb.0:
+; NOSFB-NEXT:    srai a1, a0, 63
+; NOSFB-NEXT:    xor a0, a0, a1
+; NOSFB-NEXT:    sub a0, a0, a1
+; NOSFB-NEXT:    ret
+;
+; RV64SFB-LABEL: abs_i64:
+; RV64SFB:       # %bb.0:
+; RV64SFB-NEXT:    bgez a0, .LBB35_2
+; RV64SFB-NEXT:  # %bb.1:
+; RV64SFB-NEXT:    neg a0, a0
+; RV64SFB-NEXT:  .LBB35_2:
+; RV64SFB-NEXT:    ret
+;
+; ZICOND-LABEL: abs_i64:
+; ZICOND:       # %bb.0:
+; ZICOND-NEXT:    bgez a0, .LBB35_2
+; ZICOND-NEXT:  # %bb.1:
+; ZICOND-NEXT:    neg a0, a0
+; ZICOND-NEXT:  .LBB35_2:
+; ZICOND-NEXT:    ret
+;
+; RV32SFB-LABEL: abs_i64:
+; RV32SFB:       # %bb.0:
+; RV32SFB-NEXT:    snez a2, a0
+; RV32SFB-NEXT:    add a2, a2, a1
+; RV32SFB-NEXT:    bgez a1, .LBB35_2
+; RV32SFB-NEXT:  # %bb.1:
+; RV32SFB-NEXT:    neg a0, a0
+; RV32SFB-NEXT:  .LBB35_2:
+; RV32SFB-NEXT:    bgez a1, .LBB35_4
+; RV32SFB-NEXT:  # %bb.3:
+; RV32SFB-NEXT:    neg a1, a2
+; RV32SFB-NEXT:  .LBB35_4:
+; RV32SFB-NEXT:    ret
+  %a = call i64 @llvm.abs.i64(i64 %x, i1 false)
+  ret i64 %a
+}
+declare i64 @llvm.abs.i64(i64, i1)