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Add should_skip_bounds_check to MIR Builder #153602

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5 changes: 3 additions & 2 deletions compiler/rustc_ast_lowering/src/format.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ use std::borrow::Cow;
use rustc_ast::*;
use rustc_data_structures::fx::FxIndexMap;
use rustc_hir as hir;
use rustc_middle::mir::interpret::PointerArithmetic;
use rustc_session::config::FmtDebug;
use rustc_span::{ByteSymbol, DesugaringKind, Ident, Span, Symbol, sym};

Expand Down Expand Up @@ -56,7 +57,7 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
/// Get the maximum value of int_ty. It is platform-dependent due to the byte size of isize
fn int_ty_max(&self, int_ty: IntTy) -> u128 {
match int_ty {
IntTy::Isize => self.tcx.data_layout.pointer_size().signed_int_max() as u128,
IntTy::Isize => self.tcx.target_isize_max() as u128,
IntTy::I8 => i8::MAX as u128,
IntTy::I16 => i16::MAX as u128,
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@scottmcm scottmcm Mar 20, 2026

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I still think you shouldn't need this match, by finding some method that'll do the right thing. Like if you can get to Integer the match would be replaced with https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.Integer.html#method.signed_max.

Or Size::from_bytes(int_ty.num_bytes()).signed_int_max() or something.

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@Bryntet Bryntet Mar 20, 2026

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Whoops, this is actually an unrelated change to the PR that snuck in, this isn't the actual match I added but one I noticed existed while searching for the good way to get Size, like you said, I'll ping you in a comment on the cofe where I actually changed it for this PR

IntTy::I32 => i32::MAX as u128,
Expand All @@ -68,7 +69,7 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
/// Get the maximum value of uint_ty. It is platform-dependent due to the byte size of usize
fn uint_ty_max(&self, uint_ty: UintTy) -> u128 {
match uint_ty {
UintTy::Usize => self.tcx.data_layout.pointer_size().unsigned_int_max(),
UintTy::Usize => self.tcx.target_usize_max() as u128,
UintTy::U8 => u8::MAX as u128,
UintTy::U16 => u16::MAX as u128,
UintTy::U32 => u32::MAX as u128,
Expand Down
43 changes: 41 additions & 2 deletions compiler/rustc_mir_transform/src/gvn.rs
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Expand Up @@ -91,7 +91,9 @@ use std::hash::{Hash, Hasher};

use either::Either;
use itertools::Itertools as _;
use rustc_abi::{self as abi, BackendRepr, FIRST_VARIANT, FieldIdx, Primitive, Size, VariantIdx};
use rustc_abi::{
self as abi, BackendRepr, FIRST_VARIANT, FieldIdx, Integer, Primitive, Size, VariantIdx,
};
use rustc_arena::DroplessArena;
use rustc_const_eval::const_eval::DummyMachine;
use rustc_const_eval::interpret::{
Expand All @@ -108,7 +110,7 @@ use rustc_middle::bug;
use rustc_middle::mir::interpret::{AllocRange, GlobalAlloc};
use rustc_middle::mir::visit::*;
use rustc_middle::mir::*;
use rustc_middle::ty::layout::HasTypingEnv;
use rustc_middle::ty::layout::{HasTypingEnv, IntegerExt};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_span::DUMMY_SP;
use smallvec::SmallVec;
Expand Down Expand Up @@ -1482,6 +1484,33 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
(BinOp::Eq, a, b) if a == b => self.insert_bool(true),
(BinOp::Ne, Left(a), Left(b)) => self.insert_bool(a != b),
(BinOp::Ne, a, b) if a == b => self.insert_bool(false),
// When casting from a value, and comparing with a literal
// compare the maximum value with this literal
// to see if it's possible omit the runtime check
(BinOp::Lt, Right(a), Left(b)) if self.max_value_of_cast(a).is_some_and(|a| a < b) => {
self.insert_bool(true)
}
(BinOp::Lt, Left(a), Right(b)) if self.max_value_of_cast(b).is_some_and(|b| a >= b) => {
self.insert_bool(false)
}
(BinOp::Le, Right(a), Left(b)) if self.max_value_of_cast(a).is_some_and(|a| a <= b) => {
self.insert_bool(true)
}
(BinOp::Le, Left(a), Right(b)) if self.max_value_of_cast(b).is_some_and(|b| a > b) => {
self.insert_bool(false)
}
(BinOp::Gt, Left(a), Right(b)) if self.max_value_of_cast(b).is_some_and(|b| a > b) => {
self.insert_bool(true)
}
(BinOp::Gt, Right(a), Left(b)) if self.max_value_of_cast(a).is_some_and(|a| a <= b) => {
self.insert_bool(false)
}
(BinOp::Ge, Left(a), Right(b)) if self.max_value_of_cast(b).is_some_and(|b| a >= b) => {
self.insert_bool(true)
}
(BinOp::Ge, Right(a), Left(b)) if self.max_value_of_cast(a).is_some_and(|a| a < b) => {
self.insert_bool(false)
}
_ => return None,
};

Expand All @@ -1494,6 +1523,16 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
}
}

fn max_value_of_cast(&self, value: VnIndex) -> Option<u128> {
if let Value::Cast { kind: CastKind::IntToInt, value } = self.get(value) {
self.max_value_of_cast(value)
} else if let ty::Uint(uty) = self.ty(value).kind() {
Some(Integer::from_uint_ty(&self.tcx, *uty).size().unsigned_int_max())
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@Bryntet Bryntet Mar 20, 2026

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Here is the actual Size::unsigned_int_max() call

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@Bryntet Bryntet Mar 20, 2026

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@scottmcm

} else {
None
}
}

fn simplify_cast(
&mut self,
initial_kind: &mut CastKind,
Expand Down
40 changes: 40 additions & 0 deletions tests/mir-opt/const_prop/index_cast.array.GVN.32bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,40 @@
- // MIR for `array` before GVN
+ // MIR for `array` after GVN

fn array(_1: [i32; 256], _2: u8) -> () {
debug input => _1;
debug lit => _2;
let mut _0: ();
let _3: i32;
let _4: usize;
let mut _5: u8;
let mut _6: bool;
scope 1 {
debug x => _3;
}

bb0: {
StorageLive(_3);
- StorageLive(_4);
+ nop;
StorageLive(_5);
_5 = copy _2;
- _4 = move _5 as usize (IntToInt);
+ _4 = copy _2 as usize (IntToInt);
StorageDead(_5);
- _6 = Lt(copy _4, const 256_usize);
- assert(move _6, "index out of bounds: the length is {} but the index is {}", const 256_usize, copy _4) -> [success: bb1, unwind continue];
+ _6 = const true;
+ assert(const true, "index out of bounds: the length is {} but the index is {}", const 256_usize, copy _4) -> [success: bb1, unwind continue];
}

bb1: {
_3 = copy _1[_4];
- StorageDead(_4);
+ nop;
_0 = const ();
StorageDead(_3);
return;
}
}

40 changes: 40 additions & 0 deletions tests/mir-opt/const_prop/index_cast.array.GVN.64bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,40 @@
- // MIR for `array` before GVN
+ // MIR for `array` after GVN

fn array(_1: [i32; 256], _2: u8) -> () {
debug input => _1;
debug lit => _2;
let mut _0: ();
let _3: i32;
let _4: usize;
let mut _5: u8;
let mut _6: bool;
scope 1 {
debug x => _3;
}

bb0: {
StorageLive(_3);
- StorageLive(_4);
+ nop;
StorageLive(_5);
_5 = copy _2;
- _4 = move _5 as usize (IntToInt);
+ _4 = copy _2 as usize (IntToInt);
StorageDead(_5);
- _6 = Lt(copy _4, const 256_usize);
- assert(move _6, "index out of bounds: the length is {} but the index is {}", const 256_usize, copy _4) -> [success: bb1, unwind continue];
+ _6 = const true;
+ assert(const true, "index out of bounds: the length is {} but the index is {}", const 256_usize, copy _4) -> [success: bb1, unwind continue];
}

bb1: {
_3 = copy _1[_4];
- StorageDead(_4);
+ nop;
_0 = const ();
StorageDead(_3);
return;
}
}

71 changes: 71 additions & 0 deletions tests/mir-opt/const_prop/index_cast.ge.GVN.32bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,71 @@
- // MIR for `ge` before GVN
+ // MIR for `ge` after GVN

fn ge(_1: u32) -> () {
debug input => _1;
let mut _0: ();
let _2: bool;
let mut _3: u64;
let mut _4: u32;
let mut _6: u64;
let mut _7: u32;
let mut _8: u64;
let mut _10: u64;
let mut _11: u32;
scope 1 {
debug yes => _2;
let _5: bool;
scope 2 {
debug runtime => _5;
let _9: bool;
scope 3 {
debug no => _9;
}
}
}

bb0: {
StorageLive(_2);
- StorageLive(_3);
+ nop;
StorageLive(_4);
_4 = copy _1;
- _3 = move _4 as u64 (IntToInt);
+ _3 = copy _1 as u64 (IntToInt);
StorageDead(_4);
- _2 = Ge(const U32_MAX_PLUS_ONE, move _3);
- StorageDead(_3);
+ _2 = const true;
+ nop;
StorageLive(_5);
StorageLive(_6);
StorageLive(_7);
_7 = copy _1;
- _6 = move _7 as u64 (IntToInt);
+ _6 = copy _3;
StorageDead(_7);
StorageLive(_8);
- _8 = const core::num::<impl u32>::MAX as u64 (IntToInt);
- _5 = Ge(move _6, move _8);
+ _8 = const 4294967295_u64;
+ _5 = Ge(copy _3, const 4294967295_u64);
StorageDead(_8);
StorageDead(_6);
StorageLive(_9);
StorageLive(_10);
StorageLive(_11);
_11 = copy _1;
- _10 = move _11 as u64 (IntToInt);
+ _10 = copy _3;
StorageDead(_11);
- _9 = Ge(move _10, const U32_MAX_PLUS_ONE);
+ _9 = const false;
StorageDead(_10);
_0 = const ();
StorageDead(_9);
StorageDead(_5);
StorageDead(_2);
return;
}
}

71 changes: 71 additions & 0 deletions tests/mir-opt/const_prop/index_cast.ge.GVN.64bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,71 @@
- // MIR for `ge` before GVN
+ // MIR for `ge` after GVN

fn ge(_1: u32) -> () {
debug input => _1;
let mut _0: ();
let _2: bool;
let mut _3: u64;
let mut _4: u32;
let mut _6: u64;
let mut _7: u32;
let mut _8: u64;
let mut _10: u64;
let mut _11: u32;
scope 1 {
debug yes => _2;
let _5: bool;
scope 2 {
debug runtime => _5;
let _9: bool;
scope 3 {
debug no => _9;
}
}
}

bb0: {
StorageLive(_2);
- StorageLive(_3);
+ nop;
StorageLive(_4);
_4 = copy _1;
- _3 = move _4 as u64 (IntToInt);
+ _3 = copy _1 as u64 (IntToInt);
StorageDead(_4);
- _2 = Ge(const U32_MAX_PLUS_ONE, move _3);
- StorageDead(_3);
+ _2 = const true;
+ nop;
StorageLive(_5);
StorageLive(_6);
StorageLive(_7);
_7 = copy _1;
- _6 = move _7 as u64 (IntToInt);
+ _6 = copy _3;
StorageDead(_7);
StorageLive(_8);
- _8 = const core::num::<impl u32>::MAX as u64 (IntToInt);
- _5 = Ge(move _6, move _8);
+ _8 = const 4294967295_u64;
+ _5 = Ge(copy _3, const 4294967295_u64);
StorageDead(_8);
StorageDead(_6);
StorageLive(_9);
StorageLive(_10);
StorageLive(_11);
_11 = copy _1;
- _10 = move _11 as u64 (IntToInt);
+ _10 = copy _3;
StorageDead(_11);
- _9 = Ge(move _10, const U32_MAX_PLUS_ONE);
+ _9 = const false;
StorageDead(_10);
_0 = const ();
StorageDead(_9);
StorageDead(_5);
StorageDead(_2);
return;
}
}

49 changes: 49 additions & 0 deletions tests/mir-opt/const_prop/index_cast.gt.GVN.32bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,49 @@
- // MIR for `gt` before GVN
+ // MIR for `gt` after GVN

fn gt(_1: u8) -> () {
debug input => _1;
let mut _0: ();
let _2: bool;
let mut _3: u16;
let mut _4: u8;
let mut _6: u16;
let mut _7: u8;
scope 1 {
debug yes => _2;
let _5: bool;
scope 2 {
debug no => _5;
}
}

bb0: {
StorageLive(_2);
- StorageLive(_3);
+ nop;
StorageLive(_4);
_4 = copy _1;
- _3 = move _4 as u16 (IntToInt);
+ _3 = copy _1 as u16 (IntToInt);
StorageDead(_4);
- _2 = Gt(const 256_u16, move _3);
- StorageDead(_3);
+ _2 = const true;
+ nop;
StorageLive(_5);
StorageLive(_6);
StorageLive(_7);
_7 = copy _1;
- _6 = move _7 as u16 (IntToInt);
+ _6 = copy _3;
StorageDead(_7);
- _5 = Gt(move _6, const 256_u16);
+ _5 = const false;
StorageDead(_6);
_0 = const ();
StorageDead(_5);
StorageDead(_2);
return;
}
}

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