Override carrying_mul_add in cg_llvm

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -340,6 +340,37 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                     self.const_i32(cache_type),
                 ])
             }
+            sym::carrying_mul_add => {
+                let (size, signed) = fn_args.type_at(0).int_size_and_signed(self.tcx);
+
+                let wide_llty = self.type_ix(size.bits() * 2);
+                let args = args.as_array().unwrap();
+                let [a, b, c, d] = args.map(|a| self.intcast(a.immediate(), wide_llty, signed));
+
+                let wide = if signed {
+                    let prod = self.unchecked_smul(a, b);
+                    let acc = self.unchecked_sadd(prod, c);
+                    self.unchecked_sadd(acc, d)
+                } else {
+                    let prod = self.unchecked_umul(a, b);
+                    let acc = self.unchecked_uadd(prod, c);
+                    self.unchecked_uadd(acc, d)
+                };
+
+                let narrow_llty = self.type_ix(size.bits());
+                let low = self.trunc(wide, narrow_llty);
+                let bits_const = self.const_uint(wide_llty, size.bits());
+                // No need for ashr when signed; LLVM changes it to lshr anyway.
+                let high = self.lshr(wide, bits_const);
+                // FIXME: could be `trunc nuw`, even for signed.
+                let high = self.trunc(high, narrow_llty);
+
+                let pair_llty = self.type_struct(&[narrow_llty, narrow_llty], false);
+                let pair = self.const_poison(pair_llty);
+                let pair = self.insert_value(pair, low, 0);
+                let pair = self.insert_value(pair, high, 1);
+                pair
+            }
             sym::ctlz
             | sym::ctlz_nonzero
             | sym::cttz

compiler/rustc_codegen_llvm/src/lib.rs

@@ -17,6 +17,7 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
+#![feature(slice_as_array)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 

library/core/src/intrinsics/fallback.rs

@@ -100,8 +100,8 @@ impl const CarryingMulAdd for i128 {
     fn carrying_mul_add(self, b: i128, c: i128, d: i128) -> (u128, i128) {
         let (low, high) = wide_mul_u128(self as u128, b as u128);
         let mut high = high as i128;
-        high = high.wrapping_add((self >> 127) * b);
-        high = high.wrapping_add(self * (b >> 127));
+        high = high.wrapping_add(i128::wrapping_mul(self >> 127, b));
+        high = high.wrapping_add(i128::wrapping_mul(self, b >> 127));
         let (low, carry) = u128::overflowing_add(low, c as u128);
         high = high.wrapping_add((carry as i128) + (c >> 127));
         let (low, carry) = u128::overflowing_add(low, d as u128);

library/core/tests/intrinsics.rs

@@ -153,6 +153,7 @@ fn carrying_mul_add_fallback_i32() {
 
 #[test]
 fn carrying_mul_add_fallback_u128() {
+    assert_eq!(fallback_cma::<u128>(u128::MAX, u128::MAX, 0, 0), (1, u128::MAX - 1));
     assert_eq!(fallback_cma::<u128>(1, 1, 1, 1), (3, 0));
     assert_eq!(fallback_cma::<u128>(0, 0, u128::MAX, u128::MAX), (u128::MAX - 1, 1));
     assert_eq!(
@@ -178,8 +179,17 @@ fn carrying_mul_add_fallback_u128() {
 
 #[test]
 fn carrying_mul_add_fallback_i128() {
+    assert_eq!(fallback_cma::<i128>(-1, -1, 0, 0), (1, 0));
     let r = fallback_cma::<i128>(-1, -1, -1, -1);
     assert_eq!(r, (u128::MAX, -1));
     let r = fallback_cma::<i128>(1, -1, 1, 1);
     assert_eq!(r, (1, 0));
+    assert_eq!(
+        fallback_cma::<i128>(i128::MAX, i128::MAX, i128::MAX, i128::MAX),
+        (u128::MAX, i128::MAX / 2),
+    );
+    assert_eq!(
+        fallback_cma::<i128>(i128::MIN, i128::MIN, i128::MAX, i128::MAX),
+        (u128::MAX - 1, -(i128::MIN / 2)),
+    );
 }

tests/codegen/intrinsics/carrying_mul_add.rs

@@ -0,0 +1,137 @@
+//@ revisions: RAW OPT
+//@ compile-flags: -C opt-level=1
+//@[RAW] compile-flags: -C no-prepopulate-passes
+//@[OPT] min-llvm-version: 19
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+#![feature(core_intrinsics_fallbacks)]
+
+// Note that LLVM seems to sometimes permute the order of arguments to mul and add,
+// so these tests don't check the arguments in the optimized revision.
+
+use std::intrinsics::{carrying_mul_add, fallback};
+
+// The fallbacks are emitted even when they're never used, but optimize out.
+
+// RAW: wide_mul_u128
+// OPT-NOT: wide_mul_u128
+
+// CHECK-LABEL: @cma_u8
+#[no_mangle]
+pub unsafe fn cma_u8(a: u8, b: u8, c: u8, d: u8) -> (u8, u8) {
+    // CHECK: [[A:%.+]] = zext i8 %a to i16
+    // CHECK: [[B:%.+]] = zext i8 %b to i16
+    // CHECK: [[C:%.+]] = zext i8 %c to i16
+    // CHECK: [[D:%.+]] = zext i8 %d to i16
+    // CHECK: [[AB:%.+]] = mul nuw i16
+    // RAW-SAME: [[A]], [[B]]
+    // CHECK: [[ABC:%.+]] = add nuw i16
+    // RAW-SAME: [[AB]], [[C]]
+    // CHECK: [[ABCD:%.+]] = add nuw i16
+    // RAW-SAME: [[ABC]], [[D]]
+    // CHECK: [[LOW:%.+]] = trunc i16 [[ABCD]] to i8
+    // CHECK: [[HIGHW:%.+]] = lshr i16 [[ABCD]], 8
+    // RAW: [[HIGH:%.+]] = trunc i16 [[HIGHW]] to i8
+    // OPT: [[HIGH:%.+]] = trunc nuw i16 [[HIGHW]] to i8
+    // CHECK: [[PAIR0:%.+]] = insertvalue { i8, i8 } poison, i8 [[LOW]], 0
+    // CHECK: [[PAIR1:%.+]] = insertvalue { i8, i8 } [[PAIR0]], i8 [[HIGH]], 1
+    // OPT: ret { i8, i8 } [[PAIR1]]
+    carrying_mul_add(a, b, c, d)
+}
+
+// CHECK-LABEL: @cma_u32
+#[no_mangle]
+pub unsafe fn cma_u32(a: u32, b: u32, c: u32, d: u32) -> (u32, u32) {
+    // CHECK: [[A:%.+]] = zext i32 %a to i64
+    // CHECK: [[B:%.+]] = zext i32 %b to i64
+    // CHECK: [[C:%.+]] = zext i32 %c to i64
+    // CHECK: [[D:%.+]] = zext i32 %d to i64
+    // CHECK: [[AB:%.+]] = mul nuw i64
+    // RAW-SAME: [[A]], [[B]]
+    // CHECK: [[ABC:%.+]] = add nuw i64
+    // RAW-SAME: [[AB]], [[C]]
+    // CHECK: [[ABCD:%.+]] = add nuw i64
+    // RAW-SAME: [[ABC]], [[D]]
+    // CHECK: [[LOW:%.+]] = trunc i64 [[ABCD]] to i32
+    // CHECK: [[HIGHW:%.+]] = lshr i64 [[ABCD]], 32
+    // RAW: [[HIGH:%.+]] = trunc i64 [[HIGHW]] to i32
+    // OPT: [[HIGH:%.+]] = trunc nuw i64 [[HIGHW]] to i32
+    // CHECK: [[PAIR0:%.+]] = insertvalue { i32, i32 } poison, i32 [[LOW]], 0
+    // CHECK: [[PAIR1:%.+]] = insertvalue { i32, i32 } [[PAIR0]], i32 [[HIGH]], 1
+    // OPT: ret { i32, i32 } [[PAIR1]]
+    carrying_mul_add(a, b, c, d)
+}
+
+// CHECK-LABEL: @cma_u128
+// CHECK-SAME: sret{{.+}}dereferenceable(32){{.+}}%_0,{{.+}}%a,{{.+}}%b,{{.+}}%c,{{.+}}%d
+#[no_mangle]
+pub unsafe fn cma_u128(a: u128, b: u128, c: u128, d: u128) -> (u128, u128) {
+    // CHECK: [[A:%.+]] = zext i128 %a to i256
+    // CHECK: [[B:%.+]] = zext i128 %b to i256
+    // CHECK: [[C:%.+]] = zext i128 %c to i256
+    // CHECK: [[D:%.+]] = zext i128 %d to i256
+    // CHECK: [[AB:%.+]] = mul nuw i256
+    // RAW-SAME: [[A]], [[B]]
+    // CHECK: [[ABC:%.+]] = add nuw i256
+    // RAW-SAME: [[AB]], [[C]]
+    // CHECK: [[ABCD:%.+]] = add nuw i256
+    // RAW-SAME: [[ABC]], [[D]]
+    // CHECK: [[LOW:%.+]] = trunc i256 [[ABCD]] to i128
+    // CHECK: [[HIGHW:%.+]] = lshr i256 [[ABCD]], 128
+    // RAW: [[HIGH:%.+]] = trunc i256 [[HIGHW]] to i128
+    // OPT: [[HIGH:%.+]] = trunc nuw i256 [[HIGHW]] to i128
+    // RAW: [[PAIR0:%.+]] = insertvalue { i128, i128 } poison, i128 [[LOW]], 0
+    // RAW: [[PAIR1:%.+]] = insertvalue { i128, i128 } [[PAIR0]], i128 [[HIGH]], 1
+    // OPT: store i128 [[LOW]], ptr %_0
+    // OPT: [[P1:%.+]] = getelementptr inbounds i8, ptr %_0, i64 16
+    // OPT: store i128 [[HIGH]], ptr [[P1]]
+    // CHECK: ret void
+    carrying_mul_add(a, b, c, d)
+}
+
+// CHECK-LABEL: @cma_i128
+// CHECK-SAME: sret{{.+}}dereferenceable(32){{.+}}%_0,{{.+}}%a,{{.+}}%b,{{.+}}%c,{{.+}}%d
+#[no_mangle]
+pub unsafe fn cma_i128(a: i128, b: i128, c: i128, d: i128) -> (u128, i128) {
+    // CHECK: [[A:%.+]] = sext i128 %a to i256
+    // CHECK: [[B:%.+]] = sext i128 %b to i256
+    // CHECK: [[C:%.+]] = sext i128 %c to i256
+    // CHECK: [[D:%.+]] = sext i128 %d to i256
+    // CHECK: [[AB:%.+]] = mul nsw i256
+    // RAW-SAME: [[A]], [[B]]
+    // CHECK: [[ABC:%.+]] = add nsw i256
+    // RAW-SAME: [[AB]], [[C]]
+    // CHECK: [[ABCD:%.+]] = add nsw i256
+    // RAW-SAME: [[ABC]], [[D]]
+    // CHECK: [[LOW:%.+]] = trunc i256 [[ABCD]] to i128
+    // CHECK: [[HIGHW:%.+]] = lshr i256 [[ABCD]], 128
+    // RAW: [[HIGH:%.+]] = trunc i256 [[HIGHW]] to i128
+    // OPT: [[HIGH:%.+]] = trunc nuw i256 [[HIGHW]] to i128
+    // RAW: [[PAIR0:%.+]] = insertvalue { i128, i128 } poison, i128 [[LOW]], 0
+    // RAW: [[PAIR1:%.+]] = insertvalue { i128, i128 } [[PAIR0]], i128 [[HIGH]], 1
+    // OPT: store i128 [[LOW]], ptr %_0
+    // OPT: [[P1:%.+]] = getelementptr inbounds i8, ptr %_0, i64 16
+    // OPT: store i128 [[HIGH]], ptr [[P1]]
+    // CHECK: ret void
+    carrying_mul_add(a, b, c, d)
+}
+
+// CHECK-LABEL: @fallback_cma_u32
+#[no_mangle]
+pub unsafe fn fallback_cma_u32(a: u32, b: u32, c: u32, d: u32) -> (u32, u32) {
+    // OPT-DAG: [[A:%.+]] = zext i32 %a to i64
+    // OPT-DAG: [[B:%.+]] = zext i32 %b to i64
+    // OPT-DAG: [[AB:%.+]] = mul nuw i64
+    // OPT-DAG: [[C:%.+]] = zext i32 %c to i64
+    // OPT-DAG: [[ABC:%.+]] = add nuw i64{{.+}}[[C]]
+    // OPT-DAG: [[D:%.+]] = zext i32 %d to i64
+    // OPT-DAG: [[ABCD:%.+]] = add nuw i64{{.+}}[[D]]
+    // OPT-DAG: [[LOW:%.+]] = trunc i64 [[ABCD]] to i32
+    // OPT-DAG: [[HIGHW:%.+]] = lshr i64 [[ABCD]], 32
+    // OPT-DAG: [[HIGH:%.+]] = trunc nuw i64 [[HIGHW]] to i32
+    // OPT-DAG: [[PAIR0:%.+]] = insertvalue { i32, i32 } poison, i32 [[LOW]], 0
+    // OPT-DAG: [[PAIR1:%.+]] = insertvalue { i32, i32 } [[PAIR0]], i32 [[HIGH]], 1
+    // OPT-DAG: ret { i32, i32 } [[PAIR1]]
+    fallback::CarryingMulAdd::carrying_mul_add(a, b, c, d)
+}