Use LLVM intrinsics for saturating add/sub

2 files changed, 65 insertions, 3 deletions

diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 1d7f14b02e1..f6795588441 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -757,6 +757,30 @@ impl CodegenCx<'b, 'tcx> {
         ifn!("llvm.umul.with.overflow.i64", fn(t_i64, t_i64) -> mk_struct!{t_i64, i1});
         ifn!("llvm.umul.with.overflow.i128", fn(t_i128, t_i128) -> mk_struct!{t_i128, i1});
 
+        ifn!("llvm.sadd.sat.i8", fn(t_i8, t_i8) -> t_i8);
+        ifn!("llvm.sadd.sat.i16", fn(t_i16, t_i16) -> t_i16);
+        ifn!("llvm.sadd.sat.i32", fn(t_i32, t_i32) -> t_i32);
+        ifn!("llvm.sadd.sat.i64", fn(t_i64, t_i64) -> t_i64);
+        ifn!("llvm.sadd.sat.i128", fn(t_i128, t_i128) -> t_i128);
+
+        ifn!("llvm.uadd.sat.i8", fn(t_i8, t_i8) -> t_i8);
+        ifn!("llvm.uadd.sat.i16", fn(t_i16, t_i16) -> t_i16);
+        ifn!("llvm.uadd.sat.i32", fn(t_i32, t_i32) -> t_i32);
+        ifn!("llvm.uadd.sat.i64", fn(t_i64, t_i64) -> t_i64);
+        ifn!("llvm.uadd.sat.i128", fn(t_i128, t_i128) -> t_i128);
+
+        ifn!("llvm.ssub.sat.i8", fn(t_i8, t_i8) -> t_i8);
+        ifn!("llvm.ssub.sat.i16", fn(t_i16, t_i16) -> t_i16);
+        ifn!("llvm.ssub.sat.i32", fn(t_i32, t_i32) -> t_i32);
+        ifn!("llvm.ssub.sat.i64", fn(t_i64, t_i64) -> t_i64);
+        ifn!("llvm.ssub.sat.i128", fn(t_i128, t_i128) -> t_i128);
+
+        ifn!("llvm.usub.sat.i8", fn(t_i8, t_i8) -> t_i8);
+        ifn!("llvm.usub.sat.i16", fn(t_i16, t_i16) -> t_i16);
+        ifn!("llvm.usub.sat.i32", fn(t_i32, t_i32) -> t_i32);
+        ifn!("llvm.usub.sat.i64", fn(t_i64, t_i64) -> t_i64);
+        ifn!("llvm.usub.sat.i128", fn(t_i128, t_i128) -> t_i128);
+
         ifn!("llvm.lifetime.start", fn(t_i64,i8p) -> void);
         ifn!("llvm.lifetime.end", fn(t_i64, i8p) -> void);
 
diff --git a/src/librustc_codegen_llvm/intrinsic.rs b/src/librustc_codegen_llvm/intrinsic.rs
index 201b1684fb9..58b466dbe6f 100644
--- a/src/librustc_codegen_llvm/intrinsic.rs
+++ b/src/librustc_codegen_llvm/intrinsic.rs
@@ -14,7 +14,7 @@ use type_::Type;
 use type_of::LayoutLlvmExt;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{self, LayoutOf, HasTyCtxt, Primitive};
-use rustc_codegen_ssa::common::TypeKind;
+use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
 use rustc::hir;
 use syntax::ast::{self, FloatTy};
 use syntax::symbol::Symbol;
@@ -28,7 +28,7 @@ use rustc::session::Session;
 use syntax_pos::Span;
 
 use std::cmp::Ordering;
-use std::iter;
+use std::{iter, i128, u128};
 
 fn get_simple_intrinsic(cx: &CodegenCx<'ll, '_>, name: &str) -> Option<&'ll Value> {
     let llvm_name = match name {
@@ -342,7 +342,7 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
             "bitreverse" | "add_with_overflow" | "sub_with_overflow" |
             "mul_with_overflow" | "overflowing_add" | "overflowing_sub" | "overflowing_mul" |
             "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" | "exact_div" |
-            "rotate_left" | "rotate_right" => {
+            "rotate_left" | "rotate_right" | "saturating_add" | "saturating_sub" => {
                 let ty = arg_tys[0];
                 match int_type_width_signed(ty, self) {
                     Some((width, signed)) =>
@@ -468,6 +468,44 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
                                     self.or(shift1, shift2)
                                 }
                             },
+                            "saturating_add" | "saturating_sub" => {
+                                let is_add = name == "saturating_add";
+                                let lhs = args[0].immediate();
+                                let rhs = args[1].immediate();
+                                if llvm_util::get_major_version() >= 8 {
+                                    let llvm_name = &format!("llvm.{}{}.sat.i{}",
+                                                             if signed { 's' } else { 'u' },
+                                                             if is_add { "add" } else { "sub" },
+                                                             width);
+                                    let llfn = self.get_intrinsic(llvm_name);
+                                    self.call(llfn, &[lhs, rhs], None)
+                                } else {
+                                    let llvm_name = &format!("llvm.{}{}.with.overflow.i{}",
+                                                             if signed { 's' } else { 'u' },
+                                                             if is_add { "add" } else { "sub" },
+                                                             width);
+                                    let llfn = self.get_intrinsic(llvm_name);
+                                    let pair = self.call(llfn, &[lhs, rhs], None);
+                                    let val = self.extract_value(pair, 0);
+                                    let overflow = self.extract_value(pair, 1);
+                                    let llty = self.type_ix(width);
+
+                                    let limit = if signed {
+                                        let limit_lo = self.const_uint_big(
+                                            llty, (i128::MIN >> (128 - width)) as u128);
+                                        let limit_hi = self.const_uint_big(
+                                            llty, (i128::MAX >> (128 - width)) as u128);
+                                        let neg = self.icmp(
+                                            IntPredicate::IntSLT, val, self.const_uint(llty, 0));
+                                        self.select(neg, limit_hi, limit_lo)
+                                    } else if is_add {
+                                        self.const_uint_big(llty, u128::MAX >> (128 - width))
+                                    } else {
+                                        self.const_uint(llty, 0)
+                                    };
+                                    self.select(overflow, limit, val)
+                                }
+                            },
                             _ => bug!(),
                         },
                     None => {