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assume the runtime range of align_offset

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Found when I saw code with `align_to` having extraneous checks.
This commit is contained in:
Scott McMurray 2023-05-02 17:23:54 -07:00
parent 74c4821045
commit a1e5c65aa4
3 changed files with 89 additions and 4 deletions
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tests/codegen/align-offset.rs Normal file
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// compile-flags: -O
// min-llvm-version: 15.0 (because we're using opaque pointers)
// ignore-debug (debug assertions in `slice::from_raw_parts` block optimizations)
#![crate_type = "lib"]
// CHECK-LABEL: @align8
#[no_mangle]
pub fn align8(p: *const u8) -> bool {
// CHECK: ret i1 true
p.align_offset(8) < 8
}
#[repr(align(4))]
pub struct Align4([u8; 4]);
// CHECK-LABEL: @align_to4
#[no_mangle]
pub fn align_to4(x: &[u8]) -> bool {
// CHECK: ret i1 true
let (prefix, _middle, suffix) = unsafe { x.align_to::<Align4>() };
prefix.len() < 4 && suffix.len() < 4
}
// CHECK-LABEL: @align_offset_byte_ptr(ptr{{.+}}%ptr)
#[no_mangle]
pub fn align_offset_byte_ptr(ptr: *const u8) -> usize {
// CHECK: %[[ADDR:.+]] = ptrtoint ptr %ptr to [[USIZE:i[0-9]+]]
// CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
// CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
// CHECK: %[[OFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
// Since we're offsetting a byte pointer, there's no further fixups
// CHECK-NOT: shr
// CHECK-NOT: div
// CHECK-NOT: select
// CHECK: ret [[USIZE]] %[[OFFSET]]
ptr.align_offset(32)
}
// CHECK-LABEL: @align_offset_word_slice(ptr{{.+}}align 4{{.+}}%slice.0
#[no_mangle]
pub fn align_offset_word_slice(slice: &[Align4]) -> usize {
// CHECK: %[[ADDR:.+]] = ptrtoint ptr %slice.0 to [[USIZE]]
// CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
// CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
// CHECK: %[[BOFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
// CHECK: %[[OFFSET:.+]] = lshr exact [[USIZE]] %[[BOFFSET]], 2
// Slices are known to be aligned, so we don't need the "maybe -1" path
// CHECK-NOT: select
// CHECK: ret [[USIZE]] %[[OFFSET]]
slice.as_ptr().align_offset(32)
}
// CHECK-LABEL: @align_offset_word_ptr(ptr{{.+}}%ptr
#[no_mangle]
pub fn align_offset_word_ptr(ptr: *const Align4) -> usize {
// CHECK: %[[ADDR:.+]] = ptrtoint ptr %ptr to [[USIZE]]
// CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
// CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
// CHECK: %[[BOFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
// While we can always get a *byte* offset that will work, if the original
// pointer is unaligned it might be impossible to return an *element* offset
// that will make it aligned. We want it to be a `select`, not a `br`, so
// that the assembly will be branchless.
// CHECK: %[[LOW:.+]] = and [[USIZE]] %[[ADDR]], 3
// CHECK: %[[ORIGINAL_ALIGNED:.+]] = icmp eq [[USIZE]] %[[LOW]], 0
// CHECK: %[[OFFSET:.+]] = lshr exact [[USIZE]] %[[BOFFSET]], 2
// CHECK: %[[R:.+]] = select i1 %[[ORIGINAL_ALIGNED]], [[USIZE]] %[[OFFSET]], [[USIZE]] -1
// CHECK: ret [[USIZE]] %[[R]]
ptr.align_offset(32)
}