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Use GEP inbounds for ZST and DST field offsets #122048

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merged 2 commits into from
Mar 8, 2024
Merged

Use GEP inbounds for ZST and DST field offsets #122048

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8ebd307
use GEP inbounds for ZST and DST field offsets
erikdesjardins Feb 28, 2024
e349900
add test for extern type
erikdesjardins Mar 7, 2024
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7 changes: 1 addition & 6 deletions compiler/rustc_codegen_ssa/src/mir/place.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -104,10 +104,6 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
let mut simple = || {
let llval = if offset.bytes() == 0 {
self.llval
} else if field.is_zst() {
// FIXME(erikdesjardins): it should be fine to use inbounds for ZSTs too;
// keeping this logic for now to preserve previous behavior.
bx.ptradd(self.llval, bx.const_usize(offset.bytes()))
} else {
bx.inbounds_ptradd(self.llval, bx.const_usize(offset.bytes()))
};
Expand Down Expand Up @@ -168,8 +164,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
debug!("struct_field_ptr: DST field offset: {:?}", offset);

// Adjust pointer.
// FIXME(erikdesjardins): should be able to use inbounds here too.
let ptr = bx.ptradd(self.llval, offset);
let ptr = bx.inbounds_ptradd(self.llval, offset);

PlaceRef { llval: ptr, llextra: self.llextra, layout: field, align: effective_field_align }
}
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84 changes: 84 additions & 0 deletions tests/codegen/dst-offset.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,84 @@
//! This file tests that we correctly generate GEP instructions for DST
//! field offsets.
//@ compile-flags: -C no-prepopulate-passes -Copt-level=0

#![crate_type = "lib"]

#![feature(extern_types)]

use std::ptr::addr_of;

// Hack to get the correct type for usize
// CHECK: @helper([[USIZE:i[0-9]+]] %_1)
#[no_mangle]
pub fn helper(_: usize) {
}

struct Dst<T: ?Sized> {
x: u32,
y: u8,
z: T,
}

// CHECK: @dst_dyn_trait_offset(ptr align {{[0-9]+}} [[DATA_PTR:%.+]], ptr align {{[0-9]+}} [[VTABLE_PTR:%.+]])
#[no_mangle]
pub fn dst_dyn_trait_offset(s: &Dst<dyn Drop>) -> &dyn Drop {
// The alignment of dyn trait is unknown, so we compute the offset based on align from the vtable.

// CHECK: [[SIZE_PTR:%[0-9]+]] = getelementptr inbounds {{.+}} [[VTABLE_PTR]]
// CHECK: load [[USIZE]], ptr [[SIZE_PTR]]
// CHECK: [[ALIGN_PTR:%[0-9]+]] = getelementptr inbounds {{.+}} [[VTABLE_PTR]]
// CHECK: load [[USIZE]], ptr [[ALIGN_PTR]]

// CHECK: getelementptr inbounds i8, ptr [[DATA_PTR]]
// CHECK-NEXT: insertvalue
// CHECK-NEXT: insertvalue
// CHECK-NEXT: ret
&s.z
}

// CHECK-LABEL: @dst_slice_offset
#[no_mangle]
pub fn dst_slice_offset(s: &Dst<[u16]>) -> &[u16] {
// The alignment of [u16] is known, so we generate a GEP directly.

// CHECK: start:
// CHECK-NEXT: getelementptr inbounds i8, {{.+}}, [[USIZE]] 6
// CHECK-NEXT: insertvalue
// CHECK-NEXT: insertvalue
// CHECK-NEXT: ret
&s.z
}

#[repr(packed)]
struct PackedDstSlice {
x: u32,
y: u8,
z: [u16],
}

// CHECK-LABEL: @packed_dst_slice_offset
#[no_mangle]
pub fn packed_dst_slice_offset(s: &PackedDstSlice) -> *const [u16] {
// The alignment of [u16] is known, so we generate a GEP directly.

// CHECK: start:
// CHECK-NEXT: getelementptr inbounds i8, {{.+}}, [[USIZE]] 5
// CHECK-NEXT: insertvalue
// CHECK-NEXT: insertvalue
// CHECK-NEXT: ret
addr_of!(s.z)
}

extern {
pub type Extern;
}

// CHECK-LABEL: @dst_extern
#[no_mangle]
pub fn dst_extern(s: &Dst<Extern>) -> &Extern {
// Computing the alignment of an extern type is currently unsupported and just panics.

// CHECK: call void @{{.+}}panic
&s.z
}
Comment on lines +73 to +84
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Not sure if this is what you expected...

Full IR:

@str.0 = internal unnamed_addr constant [66 x i8] c"attempted to compute the size or alignment of extern type `Extern`"

define align 1 ptr @dst_extern(ptr align 4 %s) unnamed_addr #0 {
start:
; call core::panicking::panic_nounwind
  call void @_ZN4core9panicking14panic_nounwind17h5c216434c597abbdE(ptr align 1 @str.0, i64 66) #2
  %_0 = getelementptr inbounds i8, ptr %s, i64 5
  ret ptr %_0
}

funnily enough it still generates an unreachable GEP.

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yea, as long as we have a test, whatever the behaviour, that is fine with me. I just want to protect against accidentally changing behaviour here in the future

6 changes: 3 additions & 3 deletions tests/codegen/zst-offset.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ pub fn helper(_: usize) {
// CHECK-LABEL: @scalar_layout
#[no_mangle]
pub fn scalar_layout(s: &(u64, ())) {
// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 8
// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 8
let x = &s.1;
witness(&x); // keep variable in an alloca
}
Expand All @@ -22,7 +22,7 @@ pub fn scalar_layout(s: &(u64, ())) {
// CHECK-LABEL: @scalarpair_layout
#[no_mangle]
pub fn scalarpair_layout(s: &(u64, u32, ())) {
// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 12
// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 12
let x = &s.2;
witness(&x); // keep variable in an alloca
}
Expand All @@ -34,7 +34,7 @@ pub struct U64x4(u64, u64, u64, u64);
// CHECK-LABEL: @vector_layout
#[no_mangle]
pub fn vector_layout(s: &(U64x4, ())) {
// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 32
// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 32
let x = &s.1;
witness(&x); // keep variable in an alloca
}
Expand Down
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