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Add assumes to slice length calls #122926

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Add assumes to slice length calls #122926

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14 changes: 13 additions & 1 deletion compiler/rustc_codegen_ssa/src/mir/rvalue.rs
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Expand Up @@ -753,7 +753,19 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
// use common size calculation for non zero-sized types
let cg_value = self.codegen_place(bx, place.as_ref());
cg_value.len(bx.cx())
let length = cg_value.len(bx.cx());

if bx.sess().opts.optimize != OptLevel::No {
let elem_ty = cg_value.layout.field(bx, 0);
if let Some(elem_bytes) = std::num::NonZeroU64::new(elem_ty.size.bytes()) {
let isize_max = (1_u64 << (bx.sess().target.pointer_width - 1)) - 1;
let len_max = isize_max / elem_bytes;
let limit = bx.icmp(IntPredicate::IntULE, length, bx.const_usize(len_max));
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I first tried doing this with assume(sge(len * elem_bytes, 0)), but got worse results -- I think LLVM is confused by the SGE limit compared with other positive things, even though it actually simplifies ule(x, isize::MAX) into sgt(x, -1).

bx.assume(limit);
}
}

length
}

/// Codegen an `Rvalue::AddressOf` or `Rvalue::Ref`
Expand Down
4 changes: 4 additions & 0 deletions tests/codegen/issues/issue-98294-get-mut-copy-from-slice-opt.rs
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Expand Up @@ -8,6 +8,10 @@
pub fn test(a: &mut [u8], offset: usize, bytes: &[u8]) {
// CHECK-LABEL: @test(
// CHECK-NOT: call
// CHECK: call void @llvm.assume
// CHECK-NOT: call
// CHECK: call void @llvm.assume
// CHECK-NOT: call
// CHECK: call void @llvm.memcpy
// CHECK-NOT: call
// CHECK: }
Expand Down
11 changes: 6 additions & 5 deletions tests/codegen/slice-as_chunks.rs
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Expand Up @@ -7,22 +7,23 @@
// CHECK-LABEL: @chunks4
#[no_mangle]
pub fn chunks4(x: &[u8]) -> &[[u8; 4]] {
// CHECK-NEXT: start:
// CHECK-NEXT: lshr i64 %x.1, 2
// CHECK-NOT: shl
// CHECK-NOT: mul
// CHECK-NOT: udiv
// CHECK-NOT: urem
// CHECK: ret
// CHECK: %[[NEWLEN:.+]] = lshr i64 %x.1, 2
// CHECK: %[[A:.+]] = insertvalue { ptr, i64 } poison, ptr %x.0, 0
// CHECK: %[[B:.+]] = insertvalue { ptr, i64 } %[[A]], i64 %[[NEWLEN]], 1
// CHECK: ret { ptr, i64 } %[[B]]
x.as_chunks().0
}

// CHECK-LABEL: @chunks4_with_remainder
#[no_mangle]
pub fn chunks4_with_remainder(x: &[u8]) -> (&[[u8; 4]], &[u8]) {
// CHECK-DAG: and i64 %x.1, -4
// CHECK-DAG: and i64 %x.1, [[#0x7FFFFFFFFFFFFFFC]]
// CHECK-DAG: and i64 %x.1, 3
// CHECK-DAG: lshr
// CHECK-DAG: lshr i64 %x.1, 2
// CHECK-NOT: mul
// CHECK-NOT: udiv
// CHECK-NOT: urem
Expand Down
10 changes: 6 additions & 4 deletions tests/codegen/slice-iter-fold.rs
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Expand Up @@ -2,11 +2,13 @@
#![crate_type = "lib"]

// CHECK-LABEL: @slice_fold_to_last
// CHECK-SAME: %slice.0, [[USIZE:i[0-9]+]] noundef %slice.1)
#[no_mangle]
pub fn slice_fold_to_last(slice: &[i32]) -> Option<&i32> {
// CHECK-NOT: loop
// CHECK-NOT: br
// CHECK-NOT: call
// CHECK: ret
// CHECK: %[[END:.+]] = getelementptr inbounds i32, ptr %slice.0, [[USIZE]] %slice.1
// CHECK: %[[EMPTY:.+]] = icmp eq [[USIZE]] %slice.1, 0
// CHECK: %[[LAST:.+]] = getelementptr i32, ptr %[[END]], i64 -1
// CHECK: %[[R:.+]] = select i1 %[[EMPTY]], ptr null, ptr %[[LAST]]
// CHECK: ret ptr %[[R]]
slice.iter().fold(None, |_, i| Some(i))
}
16 changes: 12 additions & 4 deletions tests/codegen/slice-iter-nonnull.rs
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Expand Up @@ -52,9 +52,13 @@ pub fn slice_iter_next_back<'a>(it: &mut std::slice::Iter<'a, u32>) -> Option<&'
// CHECK-LABEL: @slice_iter_new
// CHECK-SAME: (ptr noalias noundef nonnull {{.+}} %slice.0, {{.+}} noundef %slice.1)
#[no_mangle]
pub fn slice_iter_new(slice: &[u32]) -> std::slice::Iter<'_, u32> {
pub fn slice_iter_new(slice: &[u8]) -> std::slice::Iter<'_, u8> {
// CHECK-NOT: slice
// CHECK: %[[END:.+]] = getelementptr inbounds i32{{.+}} %slice.0{{.+}} %slice.1
// CHECK: %[[NONNEG:.+]] = icmp sgt i64 %slice.1, -1
// CHECK-NOT: slice
// CHECK: call void @llvm.assume(i1 %[[NONNEG]])
// CHECK-NOT: slice
// CHECK: %[[END:.+]] = getelementptr inbounds i8{{.+}} %slice.0{{.+}} %slice.1
// CHECK-NOT: slice
// CHECK: insertvalue {{.+}} ptr %slice.0, 0
// CHECK-NOT: slice
Expand All @@ -67,9 +71,13 @@ pub fn slice_iter_new(slice: &[u32]) -> std::slice::Iter<'_, u32> {
// CHECK-LABEL: @slice_iter_mut_new
// CHECK-SAME: (ptr noalias noundef nonnull {{.+}} %slice.0, {{.+}} noundef %slice.1)
#[no_mangle]
pub fn slice_iter_mut_new(slice: &mut [u32]) -> std::slice::IterMut<'_, u32> {
pub fn slice_iter_mut_new(slice: &mut [u8]) -> std::slice::IterMut<'_, u8> {
// CHECK-NOT: slice
// CHECK: %[[NONNEG:.+]] = icmp sgt i64 %slice.1, -1
// CHECK-NOT: slice
// CHECK: call void @llvm.assume(i1 %[[NONNEG]])
// CHECK-NOT: slice
// CHECK: %[[END:.+]] = getelementptr inbounds i32{{.+}} %slice.0{{.+}} %slice.1
// CHECK: %[[END:.+]] = getelementptr inbounds i8{{.+}} %slice.0{{.+}} %slice.1
// CHECK-NOT: slice
// CHECK: insertvalue {{.+}} ptr %slice.0, 0
// CHECK-NOT: slice
Expand Down
55 changes: 55 additions & 0 deletions tests/codegen/slice-length-limits.rs
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@@ -0,0 +1,55 @@
//@ revisions: 32BIT 64BIT
//@ compile-flags: -O
//@ min-llvm-version: 18.0
//@ [32BIT] only-32bit
//@ [64BIT] only-64bit

#![crate_type = "lib"]

// Confirm that the `assume` calls from the length allows LLVM to know that some
// math on the indices can be done without overflow risk.

// CHECK-LABEL: @slice_length_demo
#[no_mangle]
pub unsafe fn slice_length_demo(x: &[u16]) {
// 32BIT: %[[LIMIT:.+]] = icmp ult [[USIZE:i32]] %x.1, [[#0x40000000]]
// 64BIT: %[[LIMIT:.+]] = icmp ult [[USIZE:i64]] %x.1, [[#0x4000000000000000]]
// CHECK: tail call void @llvm.assume(i1 %[[LIMIT]])

// CHECK: %[[Y:.+]] = phi [[USIZE]]
// CHECK-SAME: [ 0, %start ]
// CHECK: %[[PLUS_ONE:.+]] = add nuw nsw [[USIZE]] %[[Y]], 1
// CHECK: call void @do_something([[USIZE]] noundef %[[PLUS_ONE]])
// CHECK: %[[TIMES_TWO:.+]] = shl nuw nsw [[USIZE]] %[[Y]], 1
// CHECK: call void @do_something([[USIZE]] noundef %[[TIMES_TWO]])
for y in 0..x.len() {
do_something(y + 1);
do_something(y * 2);
}
}

// CHECK-LABEL: @nested_slice_length
#[no_mangle]
pub unsafe fn nested_slice_length(x: &[f32], y: &[f32]) {
// 32BIT: %[[LIMIT:.+]] = icmp ult [[USIZE:i32]] %x.1, [[#0x20000000]]
// 64BIT: %[[LIMIT:.+]] = icmp ult [[USIZE:i64]] %x.1, [[#0x2000000000000000]]
// CHECK: tail call void @llvm.assume(i1 %[[LIMIT]])
// 32BIT: %[[LIMIT:.+]] = icmp ult [[USIZE]] %y.1, [[#0x20000000]]
// 64BIT: %[[LIMIT:.+]] = icmp ult [[USIZE]] %y.1, [[#0x2000000000000000]]
// CHECK: tail call void @llvm.assume(i1 %[[LIMIT]])

// CHECK: %[[J:.+]] = phi [[USIZE]]
// CHECK: %[[I:.+]] = phi [[USIZE]]
// CHECK-NOT: phi
// CHECK: %[[T:.+]] = add nuw nsw [[USIZE]] %[[I]], %[[J]]
// CHECK: call void @do_something([[USIZE]] noundef %[[T]])
for i in 0..x.len() {
for j in 0..y.len() {
do_something(i + j);
}
}
}

extern {
fn do_something(x: usize);
}
10 changes: 5 additions & 5 deletions tests/codegen/slice-ref-equality.rs
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Expand Up @@ -15,7 +15,7 @@ use std::num::NonZero;
// CHECK-LABEL: @is_zero_slice_long
#[no_mangle]
pub fn is_zero_slice_long(data: &[u8; 456]) -> bool {
// CHECK: %[[BCMP:.+]] = tail call i32 @{{bcmp|memcmp}}({{.+}})
// CHECK: %[[BCMP:.+]] = tail call i32 @{{bcmp|memcmp}}({{.+}} 456)
// CHECK-NEXT: %[[EQ:.+]] = icmp eq i32 %[[BCMP]], 0
// CHECK-NEXT: ret i1 %[[EQ]]
&data[..] == [0; 456]
Expand Down Expand Up @@ -48,7 +48,7 @@ pub fn is_zero_array(data: &[u8; 4]) -> bool {
#[no_mangle]
fn eq_slice_of_nested_u8(x: &[[u8; 3]], y: &[[u8; 3]]) -> bool {
// CHECK: icmp eq [[USIZE]] %1, %3
// CHECK: %[[BYTES:.+]] = mul nsw [[USIZE]] %1, 3
// CHECK: %[[BYTES:.+]] = mul nuw nsw [[USIZE]] %1, 3
// CHECK: tail call{{( noundef)?}} i32 @{{bcmp|memcmp}}(ptr
// CHECK-SAME: , [[USIZE]]{{( noundef)?}} %[[BYTES]])
x == y
Expand All @@ -60,7 +60,7 @@ fn eq_slice_of_nested_u8(x: &[[u8; 3]], y: &[[u8; 3]]) -> bool {
#[no_mangle]
fn eq_slice_of_i32(x: &[i32], y: &[i32]) -> bool {
// CHECK: icmp eq [[USIZE]] %1, %3
// CHECK: %[[BYTES:.+]] = shl nsw [[USIZE]] %1, 2
// CHECK: %[[BYTES:.+]] = shl nuw nsw [[USIZE]] %1, 2
// CHECK: tail call{{( noundef)?}} i32 @{{bcmp|memcmp}}(ptr
// CHECK-SAME: , [[USIZE]]{{( noundef)?}} %[[BYTES]])
x == y
Expand All @@ -72,7 +72,7 @@ fn eq_slice_of_i32(x: &[i32], y: &[i32]) -> bool {
#[no_mangle]
fn eq_slice_of_nonzero(x: &[NonZero<i32>], y: &[NonZero<i32>]) -> bool {
// CHECK: icmp eq [[USIZE]] %1, %3
// CHECK: %[[BYTES:.+]] = shl nsw [[USIZE]] %1, 2
// CHECK: %[[BYTES:.+]] = shl nuw nsw [[USIZE]] %1, 2
// CHECK: tail call{{( noundef)?}} i32 @{{bcmp|memcmp}}(ptr
// CHECK-SAME: , [[USIZE]]{{( noundef)?}} %[[BYTES]])
x == y
Expand All @@ -84,7 +84,7 @@ fn eq_slice_of_nonzero(x: &[NonZero<i32>], y: &[NonZero<i32>]) -> bool {
#[no_mangle]
fn eq_slice_of_option_of_nonzero(x: &[Option<NonZero<i16>>], y: &[Option<NonZero<i16>>]) -> bool {
// CHECK: icmp eq [[USIZE]] %1, %3
// CHECK: %[[BYTES:.+]] = shl nsw [[USIZE]] %1, 1
// CHECK: %[[BYTES:.+]] = shl nuw nsw [[USIZE]] %1, 1
// CHECK: tail call{{( noundef)?}} i32 @{{bcmp|memcmp}}(ptr
// CHECK-SAME: , [[USIZE]]{{( noundef)?}} %[[BYTES]])
x == y
Expand Down
2 changes: 2 additions & 0 deletions tests/codegen/vecdeque-drain.rs
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Expand Up @@ -43,6 +43,8 @@ pub fn truncate(v: &mut VecDeque<i32>, n: usize) {
// CHECK-NOT: call
// CHECK: br
// CHECK-NOT: call
// CHECK: call void @llvm.assume
// CHECK-NOT: call
// CHECK: br
// CHECK-NOT: call
// CHECK: br
Expand Down
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