Auto merge of rust-lang#130426 - workingjubilee:rollup-63xnjry, r=workingjubilee

Rollup of 5 pull requests

Successful merges:

 - rust-lang#127879 (Document futility of printing temporary pointers)
 - rust-lang#130325 (Use -0.0 in `intrinsics::simd::reduce_add_unordered`)
 - rust-lang#130336 (simplify `Build::update_existing_submodule`)
 - rust-lang#130398 (Add system libs for LLVM when cross compiling for Windows)
 - rust-lang#130420 (Register tool docs for `src/tools/build_helper`)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -2066,14 +2066,14 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
         };
     }
 
-    arith_red!(simd_reduce_add_ordered: vector_reduce_add, vector_reduce_fadd, true, add, 0.0);
+    arith_red!(simd_reduce_add_ordered: vector_reduce_add, vector_reduce_fadd, true, add, -0.0);
     arith_red!(simd_reduce_mul_ordered: vector_reduce_mul, vector_reduce_fmul, true, mul, 1.0);
     arith_red!(
         simd_reduce_add_unordered: vector_reduce_add,
         vector_reduce_fadd_reassoc,
         false,
         add,
-        0.0
+        -0.0
     );
     arith_red!(
         simd_reduce_mul_unordered: vector_reduce_mul,

compiler/rustc_llvm/build.rs

@@ -220,7 +220,10 @@ fn main() {
     let mut cmd = Command::new(&llvm_config);
     cmd.arg(llvm_link_arg).arg("--libs");
 
-    if !is_crossed {
+    // Don't link system libs if cross-compiling unless targetting Windows.
+    // On Windows system DLLs aren't linked directly, instead import libraries are used.
+    // These import libraries are independent of the host.
+    if !is_crossed || target.contains("windows") {
         cmd.arg("--system-libs");
     }
 

library/core/src/fmt/mod.rs

@@ -975,9 +975,17 @@ pub trait UpperHex {
 /// `p` formatting.
 ///
 /// The `Pointer` trait should format its output as a memory location. This is commonly presented
-/// as hexadecimal.
+/// as hexadecimal. For more information on formatters, see [the module-level documentation][module].
 ///
-/// For more information on formatters, see [the module-level documentation][module].
+/// Printing of pointers is not a reliable way to discover how Rust programs are implemented.
+/// The act of reading an address changes the program itself, and may change how the data is represented
+/// in memory, and may affect which optimizations are applied to the code.
+///
+/// The printed pointer values are not guaranteed to be stable nor unique identifiers of objects.
+/// Rust allows moving values to different memory locations, and may reuse the same memory locations
+/// for different purposes.
+///
+/// There is no guarantee that the printed value can be converted back to a pointer.
 ///
 /// [module]: ../../std/fmt/index.html
 ///

src/bootstrap/src/core/build_steps/doc.rs

@@ -1013,6 +1013,14 @@ macro_rules! tool_doc {
     }
 }
 
+// NOTE: make sure to register these in `Builder::get_step_description`.
+tool_doc!(
+    BuildHelper,
+    "src/tools/build_helper",
+    rustc_tool = false,
+    is_library = true,
+    crates = ["build_helper"]
+);
 tool_doc!(Rustdoc, "src/tools/rustdoc", crates = ["rustdoc", "rustdoc-json-types"]);
 tool_doc!(Rustfmt, "src/tools/rustfmt", crates = ["rustfmt-nightly", "rustfmt-config_proc_macro"]);
 tool_doc!(Clippy, "src/tools/clippy", crates = ["clippy_config", "clippy_utils"]);

src/bootstrap/src/core/builder.rs

@@ -944,6 +944,7 @@ impl<'a> Builder<'a> {
                 doc::Bootstrap,
                 doc::Releases,
                 doc::RunMakeSupport,
+                doc::BuildHelper,
             ),
             Kind::Dist => describe!(
                 dist::Docs,

src/bootstrap/src/lib.rs

@@ -552,11 +552,7 @@ impl Build {
             // Look for `submodule.$name.path = $path`
             // Sample output: `submodule.src/rust-installer.path src/tools/rust-installer`
             let submodule = line.split_once(' ').unwrap().1;
-            let path = Path::new(submodule);
-            // Don't update the submodule unless it's already been cloned.
-            if GitInfo::new(false, path).is_managed_git_subrepository() {
-                self.config.update_submodule(submodule);
-            }
+            self.update_existing_submodule(submodule);
         }
     }
 

tests/assembly/simd/reduce-fadd-unordered.rs

@@ -0,0 +1,29 @@
+//@ revisions: x86_64 aarch64
+//@ assembly-output: emit-asm
+//@ compile-flags: --crate-type=lib -O
+//@[aarch64] only-aarch64
+//@[x86_64] only-x86_64
+//@[x86_64] compile-flags: -Ctarget-feature=+sse3
+#![feature(portable_simd)]
+#![feature(core_intrinsics)]
+use std::intrinsics::simd as intrinsics;
+use std::simd::*;
+// Regression test for https://github.com/rust-lang/rust/issues/130028
+// This intrinsic produces much worse code if you use +0.0 instead of -0.0 because
+// +0.0 isn't as easy to algebraically reassociate, even using LLVM's reassoc attribute!
+// It would emit about an extra fadd, depending on the architecture.
+
+// CHECK-LABEL: reduce_fadd_negative_zero
+pub unsafe fn reduce_fadd_negative_zero(v: f32x4) -> f32 {
+    // x86_64: addps
+    // x86_64-NEXT: movshdup
+    // x86_64-NEXT: addss
+    // x86_64-NOT: xorps
+
+    // aarch64: faddp
+    // aarch64-NEXT: faddp
+
+    // CHECK-NOT: {{f?}}add{{p?s*}}
+    // CHECK: ret
+    intrinsics::simd_reduce_add_unordered(v)
+}