Auto merge of #99174 - scottmcm:reoptimize-layout-array, r=joshtriplett

Reoptimize layout array

This way it's one check instead of two, so hopefully (cc #99117) it'll be simpler for rustc perf too 🤞

Quick demonstration:
```rust
pub fn demo(n: usize) -> Option<Layout> {
    Layout::array::<i32>(n).ok()
}
```

Nightly: <https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=e97bf33508aa03f38968101cdeb5322d>
```nasm
	mov	rax, rdi
	mov	ecx, 4
	mul	rcx
	seto	cl
	movabs	rdx, 9223372036854775805
	xor	esi, esi
	cmp	rax, rdx
	setb	sil
	shl	rsi, 2
	xor	edx, edx
	test	cl, cl
	cmove	rdx, rsi
	ret
```

This PR (note no `mul`, in addition to being much shorter):
```nasm
	xor	edx, edx
	lea	rax, [4*rcx]
	shr	rcx, 61
	sete	dl
	shl	rdx, 2
	ret
```

This is built atop `@CAD97` 's #99136; the new changes are cb8aba66ef6a0e17f08a0574e4820653e31b45a0.

I added a bunch more tests for `Layout::from_size_align` and `Layout::array` too.

1 files changed, 31 insertions, 0 deletions

diff --git a/src/test/codegen/layout-size-checks.rs b/src/test/codegen/layout-size-checks.rs
new file mode 100644
index 00000000000..d067cc10a94
--- /dev/null
+++ b/src/test/codegen/layout-size-checks.rs
@@ -0,0 +1,31 @@
+// compile-flags: -O
+// only-x86_64
+// ignore-debug: the debug assertions get in the way
+
+#![crate_type = "lib"]
+
+use std::alloc::Layout;
+
+type RGB48 = [u16; 3];
+
+// CHECK-LABEL: @layout_array_rgb48
+#[no_mangle]
+pub fn layout_array_rgb48(n: usize) -> Layout {
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    // CHECK: icmp ugt i64 %n, 1537228672809129301
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    // CHECK: mul nuw nsw i64 %n, 6
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    Layout::array::<RGB48>(n).unwrap()
+}
+
+// CHECK-LABEL: @layout_array_i32
+#[no_mangle]
+pub fn layout_array_i32(n: usize) -> Layout {
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    // CHECK: icmp ugt i64 %n, 2305843009213693951
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    // CHECK: shl nuw nsw i64 %n, 2
+    // CHECK-NOT: llvm.umul.with.overflow.i64
+    Layout::array::<i32>(n).unwrap()
+}