Merge pull request #2051 from cfallin/aarch64-add-negative-imm

Aarch64 codegen quality: handle add-negative-imm as subtract.

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -424,6 +424,35 @@ pub(crate) fn put_input_in_rse_imm12<C: LowerCtx<I = Inst>>(
     ResultRSEImm12::from_rse(put_input_in_rse(ctx, input, narrow_mode))
 }
 
+/// Like `put_input_in_rse_imm12` above, except is allowed to negate the
+/// argument (assuming a two's-complement representation with the given bit
+/// width) if this allows use of 12-bit immediate. Used to flip `add`s with
+/// negative immediates to `sub`s (and vice-versa).
+pub(crate) fn put_input_in_rse_imm12_maybe_negated<C: LowerCtx<I = Inst>>(
+    ctx: &mut C,
+    input: InsnInput,
+    twos_complement_bits: usize,
+    narrow_mode: NarrowValueMode,
+) -> (ResultRSEImm12, bool) {
+    assert!(twos_complement_bits <= 64);
+    if let Some(imm_value) = input_to_const(ctx, input) {
+        if let Some(i) = Imm12::maybe_from_u64(imm_value) {
+            return (ResultRSEImm12::Imm12(i), false);
+        }
+        let sign_extended =
+            ((imm_value as i64) << (64 - twos_complement_bits)) >> (64 - twos_complement_bits);
+        let inverted = sign_extended.wrapping_neg();
+        if let Some(i) = Imm12::maybe_from_u64(inverted as u64) {
+            return (ResultRSEImm12::Imm12(i), true);
+        }
+    }
+
+    (
+        ResultRSEImm12::from_rse(put_input_in_rse(ctx, input, narrow_mode)),
+        false,
+    )
+}
+
 pub(crate) fn put_input_in_rs_immlogic<C: LowerCtx<I = Inst>>(
     ctx: &mut C,
     input: InsnInput,

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -60,8 +60,17 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
             let ty = ty.unwrap();
             if ty_bits(ty) < 128 {
-                let rm = put_input_in_rse_imm12(ctx, inputs[1], NarrowValueMode::None);
-                let alu_op = choose_32_64(ty, ALUOp::Add32, ALUOp::Add64);
+                let (rm, negated) = put_input_in_rse_imm12_maybe_negated(
+                    ctx,
+                    inputs[1],
+                    ty_bits(ty),
+                    NarrowValueMode::None,
+                );
+                let alu_op = if !negated {
+                    choose_32_64(ty, ALUOp::Add32, ALUOp::Add64)
+                } else {
+                    choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64)
+                };
                 ctx.emit(alu_inst_imm12(alu_op, rd, rn, rm));
             } else {
                 let rm = put_input_in_reg(ctx, inputs[1], NarrowValueMode::None);
@@ -79,8 +88,17 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
             let ty = ty.unwrap();
             if ty_bits(ty) < 128 {
-                let rm = put_input_in_rse_imm12(ctx, inputs[1], NarrowValueMode::None);
-                let alu_op = choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64);
+                let (rm, negated) = put_input_in_rse_imm12_maybe_negated(
+                    ctx,
+                    inputs[1],
+                    ty_bits(ty),
+                    NarrowValueMode::None,
+                );
+                let alu_op = if !negated {
+                    choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64)
+                } else {
+                    choose_32_64(ty, ALUOp::Add32, ALUOp::Add64)
+                };
                 ctx.emit(alu_inst_imm12(alu_op, rd, rn, rm));
             } else {
                 let rm = put_input_in_reg(ctx, inputs[1], NarrowValueMode::None);

cranelift/filetests/filetests/vcode/aarch64/arithmetic.clif

@@ -380,3 +380,45 @@ block0(v0: i32, v1: i32):
 ; nextln:  mov sp, fp
 ; nextln:  ldp fp, lr, [sp], #16
 ; nextln:  ret
+
+function %f26(i32) -> i32 {
+block0(v0: i32):
+  v1 = iconst.i32 -1
+  v2 = iadd.i32 v0, v1
+  return v2
+}
+
+; check:  stp fp, lr, [sp, #-16]!
+; nextln:  mov fp, sp
+; nextln:  sub w0, w0, #1
+; nextln:  mov sp, fp
+; nextln:  ldp fp, lr, [sp], #16
+; nextln:  ret
+
+function %f27(i32) -> i32 {
+block0(v0: i32):
+  v1 = iconst.i32 -1
+  v2 = isub.i32 v0, v1
+  return v2
+}
+
+; check:  stp fp, lr, [sp, #-16]!
+; nextln:  mov fp, sp
+; nextln:  add w0, w0, #1
+; nextln:  mov sp, fp
+; nextln:  ldp fp, lr, [sp], #16
+; nextln:  ret
+
+function %f28(i64) -> i64 {
+block0(v0: i64):
+  v1 = iconst.i64 -1
+  v2 = isub.i64 v0, v1
+  return v2
+}
+
+; check:  stp fp, lr, [sp, #-16]!
+; nextln:  mov fp, sp
+; nextln:  add x0, x0, #1
+; nextln:  mov sp, fp
+; nextln:  ldp fp, lr, [sp], #16
+; nextln:  ret