[NVPTX] Lower integer absolute value idiom to abs instruction.

[jlebar]

Summary

Previously we lowered it literally, to shifts and xors.

[tra]

llvm/lib/Target/NVPTX/NVPTXInstrInfo.td

490 ↗

(On Diff #84424)

Perhaps add a link describing this way of calculating abs().
http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs

llvm/trunk/lib/Target/NVPTX/NVPTXInstrInfo.td

 defm SDIV : I3<"div.s", sdiv>;
 defm UDIV : I3<"div.u", udiv>;
 
 // The ri versions of rem.s and rem.u won't be selected; DAGCombiner::visitSREM
 // will lower it.
 defm SREM : I3<"rem.s", srem>;
 defm UREM : I3<"rem.u", urem>;
 
+// Integer absolute value.  NumBits should be one minus the bit width of RC.
+// This idiom implements the algorithm at
+// http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs.
+multiclass ABS<RegisterClass RC, int NumBits, string SizeName> {
+  def : NVPTXInst<(outs RC:$dst), (ins RC:$a),
+                  !strconcat("abs", SizeName, " \t$dst, $a;"),
+                  [(set RC:$dst, (xor (add (sra RC:$a, (i32 NumBits)), RC:$a),
+                                      (sra RC:$a, (i32 NumBits))))]>;
+}
+defm ABS_16 : ABS<Int16Regs, 15, ".s16">;
+defm ABS_32 : ABS<Int32Regs, 31, ".s32">;
+defm ABS_64 : ABS<Int64Regs, 63, ".s64">;
 
 //
 // Wide multiplication
 //
 def MULWIDES64 :
   NVPTXInst<(outs Int64Regs:$dst), (ins Int32Regs:$a, Int32Regs:$b),
             "mul.wide.s32 \t$dst, $a, $b;", []>;
 def MULWIDES64Imm :

llvm/trunk/test/CodeGen/NVPTX/idioms.ll

+; Check that various LLVM idioms get lowered to NVPTX as expected.
+
+; RUN: llc < %s -march=nvptx -mcpu=sm_20 | FileCheck %s
+; RUN: llc < %s -march=nvptx64 -mcpu=sm_20 | FileCheck %s
+
+; CHECK-LABEL: abs_i16(
+define i16 @abs_i16(i16 %a) {
+; CHECK: abs.s16
+  %neg = sub i16 0, %a
+  %abs.cond = icmp sge i16 %a, 0
+  %abs = select i1 %abs.cond, i16 %a, i16 %neg
+  ret i16 %abs
+}
+
+; CHECK-LABEL: abs_i32(
+define i32 @abs_i32(i32 %a) {
+; CHECK: abs.s32
+  %neg = sub i32 0, %a
+  %abs.cond = icmp sge i32 %a, 0
+  %abs = select i1 %abs.cond, i32 %a, i32 %neg
+  ret i32 %abs
+}
+
+; CHECK-LABEL: abs_i64(
+define i64 @abs_i64(i64 %a) {
+; CHECK: abs.s64
+  %neg = sub i64 0, %a
+  %abs.cond = icmp sge i64 %a, 0
+  %abs = select i1 %abs.cond, i64 %a, i64 %neg
+  ret i64 %abs
+}