Avoid overflow in GVN constant indexing.

compiler/rustc_mir_transform/src/gvn.rs

@@ -644,12 +644,10 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             {
                 if let Some(offset) = self.evaluated[idx].as_ref()
                     && let Ok(offset) = self.ecx.read_target_usize(offset)
+                    && let Some(min_length) = offset.checked_add(1)
                 {
-                    projection.to_mut()[i] = ProjectionElem::ConstantIndex {
+                    projection.to_mut()[i] =
-                        offset,
+                        ProjectionElem::ConstantIndex { offset, min_length, from_end: false };
-                        min_length: offset + 1,
-                        from_end: false,
-                    };
                 } else if let Some(new_idx) = self.try_as_local(idx, location) {
                     projection.to_mut()[i] = ProjectionElem::Index(new_idx);
                     self.reused_locals.insert(new_idx);

tests/mir-opt/gvn.constant_index_overflow.GVN.panic-abort.diff

@@ -0,0 +1,104 @@
+- // MIR for `constant_index_overflow` before GVN
++ // MIR for `constant_index_overflow` after GVN
+  
+  fn constant_index_overflow(_1: &[T]) -> () {
+      debug x => _1;
+      let mut _0: ();
+      let _2: usize;
+      let mut _4: bool;
+      let mut _5: usize;
+      let mut _6: usize;
+      let mut _7: &[T];
+      let _8: usize;
+      let mut _9: usize;
+      let mut _10: bool;
+      let _11: usize;
+      let mut _12: usize;
+      let mut _13: bool;
+      let mut _14: T;
+      scope 1 {
+          debug a => _2;
+          let _3: T;
+          scope 2 {
+              debug b => _3;
+          }
+      }
+  
+      bb0: {
+-         StorageLive(_2);
+-         _2 = const _ as usize (IntToInt);
++         nop;
++         _2 = const usize::MAX;
+          StorageLive(_3);
+          StorageLive(_4);
+          StorageLive(_5);
+-         _5 = _2;
++         _5 = const usize::MAX;
+          StorageLive(_6);
+          StorageLive(_7);
+          _7 = &(*_1);
+          _6 = core::slice::<impl [T]>::len(move _7) -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          StorageDead(_7);
+-         _4 = Lt(move _5, move _6);
++         _4 = Lt(const usize::MAX, move _6);
+          switchInt(move _4) -> [0: bb4, otherwise: bb2];
+      }
+  
+      bb2: {
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_8);
+-         _8 = _2;
++         _8 = const usize::MAX;
+          _9 = Len((*_1));
+-         _10 = Lt(_8, _9);
+-         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> [success: bb3, unwind unreachable];
++         _10 = Lt(const usize::MAX, _9);
++         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, const usize::MAX) -> [success: bb3, unwind unreachable];
+      }
+  
+      bb3: {
+-         _3 = (*_1)[_8];
++         _3 = (*_1)[_2];
+          StorageDead(_8);
+          goto -> bb6;
+      }
+  
+      bb4: {
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_11);
+          _11 = const 0_usize;
+          _12 = Len((*_1));
+-         _13 = Lt(_11, _12);
+-         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb5, unwind unreachable];
++         _13 = Lt(const 0_usize, _12);
++         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 0_usize) -> [success: bb5, unwind unreachable];
+      }
+  
+      bb5: {
+-         _3 = (*_1)[_11];
++         _3 = (*_1)[0 of 1];
+          StorageDead(_11);
+          goto -> bb6;
+      }
+  
+      bb6: {
+          StorageDead(_4);
+          StorageLive(_14);
+          _14 = _3;
+          _0 = opaque::<T>(move _14) -> [return: bb7, unwind unreachable];
+      }
+  
+      bb7: {
+          StorageDead(_14);
+          StorageDead(_3);
+-         StorageDead(_2);
++         nop;
+          return;
+      }
+  }
+  

tests/mir-opt/gvn.constant_index_overflow.GVN.panic-unwind.diff

@@ -0,0 +1,104 @@
+- // MIR for `constant_index_overflow` before GVN
++ // MIR for `constant_index_overflow` after GVN
+  
+  fn constant_index_overflow(_1: &[T]) -> () {
+      debug x => _1;
+      let mut _0: ();
+      let _2: usize;
+      let mut _4: bool;
+      let mut _5: usize;
+      let mut _6: usize;
+      let mut _7: &[T];
+      let _8: usize;
+      let mut _9: usize;
+      let mut _10: bool;
+      let _11: usize;
+      let mut _12: usize;
+      let mut _13: bool;
+      let mut _14: T;
+      scope 1 {
+          debug a => _2;
+          let _3: T;
+          scope 2 {
+              debug b => _3;
+          }
+      }
+  
+      bb0: {
+-         StorageLive(_2);
+-         _2 = const _ as usize (IntToInt);
++         nop;
++         _2 = const usize::MAX;
+          StorageLive(_3);
+          StorageLive(_4);
+          StorageLive(_5);
+-         _5 = _2;
++         _5 = const usize::MAX;
+          StorageLive(_6);
+          StorageLive(_7);
+          _7 = &(*_1);
+          _6 = core::slice::<impl [T]>::len(move _7) -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
+          StorageDead(_7);
+-         _4 = Lt(move _5, move _6);
++         _4 = Lt(const usize::MAX, move _6);
+          switchInt(move _4) -> [0: bb4, otherwise: bb2];
+      }
+  
+      bb2: {
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_8);
+-         _8 = _2;
++         _8 = const usize::MAX;
+          _9 = Len((*_1));
+-         _10 = Lt(_8, _9);
+-         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> [success: bb3, unwind continue];
++         _10 = Lt(const usize::MAX, _9);
++         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, const usize::MAX) -> [success: bb3, unwind continue];
+      }
+  
+      bb3: {
+-         _3 = (*_1)[_8];
++         _3 = (*_1)[_2];
+          StorageDead(_8);
+          goto -> bb6;
+      }
+  
+      bb4: {
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_11);
+          _11 = const 0_usize;
+          _12 = Len((*_1));
+-         _13 = Lt(_11, _12);
+-         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb5, unwind continue];
++         _13 = Lt(const 0_usize, _12);
++         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 0_usize) -> [success: bb5, unwind continue];
+      }
+  
+      bb5: {
+-         _3 = (*_1)[_11];
++         _3 = (*_1)[0 of 1];
+          StorageDead(_11);
+          goto -> bb6;
+      }
+  
+      bb6: {
+          StorageDead(_4);
+          StorageLive(_14);
+          _14 = _3;
+          _0 = opaque::<T>(move _14) -> [return: bb7, unwind continue];
+      }
+  
+      bb7: {
+          StorageDead(_14);
+          StorageDead(_3);
+-         StorageDead(_2);
++         nop;
+          return;
+      }
+  }
+  

tests/mir-opt/gvn.rs

@@ -609,6 +609,22 @@ fn indirect_static() {
     })
 }
 
+/// Verify that having constant index `u64::MAX` does not yield to an overflow in rustc.
+fn constant_index_overflow<T: Copy>(x: &[T]) {
+    // CHECK-LABEL: fn constant_index_overflow(
+    // CHECK: debug a => [[a:_.*]];
+    // CHECK: debug b => [[b:_.*]];
+    // CHECK: [[a]] = const usize::MAX;
+    // CHECK-NOT: = (*_1)[{{.*}} of 0];
+    // CHECK: [[b]] = (*_1)[[[a]]];
+    // CHECK-NOT: = (*_1)[{{.*}} of 0];
+    // CHECK: [[b]] = (*_1)[0 of 1];
+    // CHECK-NOT: = (*_1)[{{.*}} of 0];
+    let a = u64::MAX as usize;
+    let b = if a < x.len() { x[a] } else { x[0] };
+    opaque(b)
+}
+
 fn main() {
     subexpression_elimination(2, 4, 5);
     wrap_unwrap(5);
@@ -627,6 +643,7 @@ fn main() {
     repeat();
     fn_pointers();
     indirect_static();
+    constant_index_overflow(&[5, 3]);
 }
 
 #[inline(never)]
@@ -653,3 +670,4 @@ fn identity<T>(x: T) -> T {
 // EMIT_MIR gvn.repeat.GVN.diff
 // EMIT_MIR gvn.fn_pointers.GVN.diff
 // EMIT_MIR gvn.indirect_static.GVN.diff
+// EMIT_MIR gvn.constant_index_overflow.GVN.diff