Auto merge of #73453 - erikdesjardins:tuplayout, r=eddyb

Ignore ZST offsets when deciding whether to use Scalar/ScalarPair layout

This is important because Scalar/ScalarPair layout previously would not be used if any ZST had nonzero offset.
For example, before this change, only `((), u128)` would be laid out like `u128`, not `(u128, ())`.

Fixes #63244

2 files changed, 79 insertions, 0 deletions

diff --git a/src/test/codegen/tuple-layout-opt.rs b/src/test/codegen/tuple-layout-opt.rs
new file mode 100644
index 00000000000..e86c75f3f48
--- /dev/null
+++ b/src/test/codegen/tuple-layout-opt.rs
@@ -0,0 +1,36 @@
+// ignore-emscripten
+// compile-flags: -C no-prepopulate-passes
+
+// Test that tuples get optimized layout, in particular with a ZST in the last field (#63244)
+
+#![crate_type="lib"]
+
+type ScalarZstLast = (u128, ());
+// CHECK: define i128 @test_ScalarZstLast(i128 %_1)
+#[no_mangle]
+pub fn test_ScalarZstLast(_: ScalarZstLast) -> ScalarZstLast { loop {} }
+
+type ScalarZstFirst = ((), u128);
+// CHECK: define i128 @test_ScalarZstFirst(i128 %_1)
+#[no_mangle]
+pub fn test_ScalarZstFirst(_: ScalarZstFirst) -> ScalarZstFirst { loop {} }
+
+type ScalarPairZstLast = (u8, u128, ());
+// CHECK: define { i128, i8 } @test_ScalarPairZstLast(i128 %_1.0, i8 %_1.1)
+#[no_mangle]
+pub fn test_ScalarPairZstLast(_: ScalarPairZstLast) -> ScalarPairZstLast { loop {} }
+
+type ScalarPairZstFirst = ((), u8, u128);
+// CHECK: define { i8, i128 } @test_ScalarPairZstFirst(i8 %_1.0, i128 %_1.1)
+#[no_mangle]
+pub fn test_ScalarPairZstFirst(_: ScalarPairZstFirst) -> ScalarPairZstFirst { loop {} }
+
+type ScalarPairLotsOfZsts = ((), u8, (), u128, ());
+// CHECK: define { i128, i8 } @test_ScalarPairLotsOfZsts(i128 %_1.0, i8 %_1.1)
+#[no_mangle]
+pub fn test_ScalarPairLotsOfZsts(_: ScalarPairLotsOfZsts) -> ScalarPairLotsOfZsts { loop {} }
+
+type ScalarPairLottaNesting = (((), ((), u8, (), u128, ())), ());
+// CHECK: define { i128, i8 } @test_ScalarPairLottaNesting(i128 %_1.0, i8 %_1.1)
+#[no_mangle]
+pub fn test_ScalarPairLottaNesting(_: ScalarPairLottaNesting) -> ScalarPairLottaNesting { loop {} }
diff --git a/src/test/codegen/zst-offset.rs b/src/test/codegen/zst-offset.rs
new file mode 100644
index 00000000000..0c015fca325
--- /dev/null
+++ b/src/test/codegen/zst-offset.rs
@@ -0,0 +1,43 @@
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(repr_simd)]
+
+// Hack to get the correct size for the length part in slices
+// CHECK: @helper([[USIZE:i[0-9]+]] %_1)
+#[no_mangle]
+pub fn helper(_: usize) {
+}
+
+// Check that we correctly generate a GEP for a ZST that is not included in Scalar layout
+// CHECK-LABEL: @scalar_layout
+#[no_mangle]
+pub fn scalar_layout(s: &(u64, ())) {
+// CHECK: [[X0:%[0-9]+]] = bitcast i64* %s to i8*
+// CHECK-NEXT: [[X1:%[0-9]+]] = getelementptr i8, i8* [[X0]], [[USIZE]] 8
+    let x = &s.1;
+    &x; // keep variable in an alloca
+}
+
+// Check that we correctly generate a GEP for a ZST that is not included in ScalarPair layout
+// CHECK-LABEL: @scalarpair_layout
+#[no_mangle]
+pub fn scalarpair_layout(s: &(u64, u32, ())) {
+// CHECK: [[X0:%[0-9]+]] = bitcast { i64, i32 }* %s to i8*
+// CHECK-NEXT: [[X1:%[0-9]+]] = getelementptr i8, i8* [[X0]], [[USIZE]] 12
+    let x = &s.2;
+    &x; // keep variable in an alloca
+}
+
+#[repr(simd)]
+pub struct U64x4(u64, u64, u64, u64);
+
+// Check that we correctly generate a GEP for a ZST that is not included in Vector layout
+// CHECK-LABEL: @vector_layout
+#[no_mangle]
+pub fn vector_layout(s: &(U64x4, ())) {
+// CHECK: [[X0:%[0-9]+]] = bitcast <4 x i64>* %s to i8*
+// CHECK-NEXT: [[X1:%[0-9]+]] = getelementptr i8, i8* [[X0]], [[USIZE]] 32
+    let x = &s.1;
+    &x; // keep variable in an alloca
+}