Auto merge of #83698 - erikdesjardins:undefconst, r=RalfJung,oli-obk

Use undef for uninitialized bytes in constants

Fixes #83657

This generates good code when the const is fully uninit, e.g.

```rust
#[no_mangle]
pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
    const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
    M
}
```
generates
```asm
fully_uninit:
	ret
```

as you would expect.

There is no improvement, however, when it's partially uninit, e.g.

```rust
pub struct PartiallyUninit {
    x: u64,
    y: MaybeUninit<[u8; 10]>
}

#[no_mangle]
pub const fn partially_uninit() -> PartiallyUninit {
    const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeefcafe, y: MaybeUninit::uninit() };
    X
}
```
generates
```asm
partially_uninit:
	mov	rax, rdi
	mov	rcx, qword ptr [rip + .L__unnamed_1+16]
	mov	qword ptr [rdi + 16], rcx
	movups	xmm0, xmmword ptr [rip + .L__unnamed_1]
	movups	xmmword ptr [rdi], xmm0
	ret

.L__unnamed_1:
	.asciz	"\376\312\357\276\255\336\000"
	.zero	16
	.size	.L__unnamed_1, 24
```
which copies a bunch of zeros in place of the undef bytes, the same as before this change.

Edit: generating partially-undef constants isn't viable at the moment anyways due to #84565, so it's disabled

2 files changed, 75 insertions, 0 deletions

diff --git a/src/test/codegen/uninit-consts-allow-partially-uninit.rs b/src/test/codegen/uninit-consts-allow-partially-uninit.rs
new file mode 100644
index 00000000000..f7420e4126e
--- /dev/null
+++ b/src/test/codegen/uninit-consts-allow-partially-uninit.rs
@@ -0,0 +1,35 @@
+// compile-flags: -C no-prepopulate-passes -Z partially_uninit_const_threshold=1024
+
+// Like uninit-consts.rs, but tests that we correctly generate partially-uninit consts
+// when the (disabled by default) partially_uninit_const_threshold flag is used.
+
+#![crate_type = "lib"]
+
+use std::mem::MaybeUninit;
+
+pub struct PartiallyUninit {
+    x: u32,
+    y: MaybeUninit<[u8; 10]>
+}
+
+// This should be partially undef.
+// CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"\EF\BE\AD\DE", [12 x i8] undef }>, align 4
+
+// This shouldn't contain undef, since it's larger than the 1024 byte limit.
+// CHECK: [[UNINIT_PADDING_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
+
+// CHECK-LABEL: @partially_uninit
+#[no_mangle]
+pub const fn partially_uninit() -> PartiallyUninit {
+    const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeef, y: MaybeUninit::uninit() };
+    // CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [4 x i8], [12 x i8] }>, <{ [4 x i8], [12 x i8] }>* [[PARTIALLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 16, i1 false)
+    X
+}
+
+// CHECK-LABEL: @uninit_padding_huge
+#[no_mangle]
+pub const fn uninit_padding_huge() -> [(u32, u8); 4096] {
+    const X: [(u32, u8); 4096] = [(123, 45); 4096];
+    // CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [32768 x i8] }>, <{ [32768 x i8] }>* [[UNINIT_PADDING_HUGE]], i32 0, i32 0, i32 0), i{{(32|64)}} 32768, i1 false)
+    X
+}
diff --git a/src/test/codegen/uninit-consts.rs b/src/test/codegen/uninit-consts.rs
new file mode 100644
index 00000000000..c4c21e03f16
--- /dev/null
+++ b/src/test/codegen/uninit-consts.rs
@@ -0,0 +1,40 @@
+// compile-flags: -C no-prepopulate-passes
+
+// Check that we use undef (and not zero) for uninitialized bytes in constants.
+
+#![crate_type = "lib"]
+
+use std::mem::MaybeUninit;
+
+pub struct PartiallyUninit {
+    x: u32,
+    y: MaybeUninit<[u8; 10]>
+}
+
+// CHECK: [[FULLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [10 x i8] }> undef
+// CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [16 x i8] }> <{ [16 x i8] c"\EF\BE\AD\DE\00\00\00\00\00\00\00\00\00\00\00\00" }>, align 4
+// CHECK: [[FULLY_UNINIT_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [16384 x i8] }> undef
+
+// CHECK-LABEL: @fully_uninit
+#[no_mangle]
+pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
+    const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
+    // CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 1 %1, i8* align 1 getelementptr inbounds (<{ [10 x i8] }>, <{ [10 x i8] }>* [[FULLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 10, i1 false)
+    M
+}
+
+// CHECK-LABEL: @partially_uninit
+#[no_mangle]
+pub const fn partially_uninit() -> PartiallyUninit {
+    const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeef, y: MaybeUninit::uninit() };
+    // CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [16 x i8] }>, <{ [16 x i8] }>* [[PARTIALLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 16, i1 false)
+    X
+}
+
+// CHECK-LABEL: @fully_uninit_huge
+#[no_mangle]
+pub const fn fully_uninit_huge() -> MaybeUninit<[u32; 4096]> {
+    const F: MaybeUninit<[u32; 4096]> = MaybeUninit::uninit();
+    // CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [16384 x i8] }>, <{ [16384 x i8] }>* [[FULLY_UNINIT_HUGE]], i32 0, i32 0, i32 0), i{{(32|64)}} 16384, i1 false)
+    F
+}