Rollup merge of #114382 - scottmcm:compare-bytes-intrinsic, r=cjgillot

Add a new `compare_bytes` intrinsic instead of calling `memcmp` directly

As discussed in #113435, this lets the backends be the place that can have the "don't call the function if n == 0" logic, if it's needed for the target.  (I didn't actually *add* those checks, though, since as I understood it we didn't actually need them on known targets?)

Doing this also let me make it `const` (unstable), which I don't think `extern "C" fn memcmp` can be.

cc `@RalfJung` `@Amanieu`

1 files changed, 34 insertions, 0 deletions

diff --git a/tests/codegen/intrinsics/compare_bytes.rs b/tests/codegen/intrinsics/compare_bytes.rs
new file mode 100644
index 00000000000..e69224d818c
--- /dev/null
+++ b/tests/codegen/intrinsics/compare_bytes.rs
@@ -0,0 +1,34 @@
+// revisions: INT32 INT16
+// compile-flags: -O
+// [INT32] ignore-16bit
+// [INT16] only-16bit
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::compare_bytes;
+
+#[no_mangle]
+// CHECK-LABEL: @bytes_cmp(
+pub unsafe fn bytes_cmp(a: *const u8, b: *const u8, n: usize) -> i32 {
+    // INT32: %[[TEMP:.+]] = tail call i32 @memcmp(ptr %a, ptr %b, {{i32|i64}} %n)
+    // INT32-NOT: sext
+    // INT32: ret i32 %[[TEMP]]
+
+    // INT16: %[[TEMP1:.+]] = tail call i16 @memcmp(ptr %a, ptr %b, i16 %n)
+    // INT16: %[[TEMP2:.+]] = sext i16 %[[TEMP1]] to i32
+    // INT16: ret i32 %[[TEMP2]]
+    compare_bytes(a, b, n)
+}
+
+// Ensure that, even though there's an `sext` emitted by the intrinsic,
+// that doesn't end up pessiming checks against zero.
+#[no_mangle]
+// CHECK-LABEL: @bytes_eq(
+pub unsafe fn bytes_eq(a: *const u8, b: *const u8, n: usize) -> bool {
+    // CHECK: call {{.+}} @{{bcmp|memcmp}}(ptr %a, ptr %b, {{i16|i32|i64}} %n)
+    // CHECK-NOT: sext
+    // INT32: icmp eq i32
+    // INT16: icmp eq i16
+    compare_bytes(a, b, n) == 0_i32
+}