Eliminate some other bound checks when index comes from an enum

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -327,13 +327,29 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 if er.end != er.start
                                     && scalar.valid_range.end() > scalar.valid_range.start()
                                 {
-                                    // We want `table[e as usize]` to not
+                                    // We want `table[e as usize ± k]` to not
                                     // have bound checks, and this is the most
-                                    // convenient place to put the `assume`.
-                                    let ll_t_in_const =
+                                    // convenient place to put the `assume`s.
+                                    if *scalar.valid_range.start() > 0 {
+                                        let enum_value_lower_bound = bx
+                                            .cx()
+                                            .const_uint_big(ll_t_in, *scalar.valid_range.start());
+                                        let cmp_start = bx.icmp(
+                                            IntPredicate::IntUGE,
+                                            llval,
+                                            enum_value_lower_bound,
+                                        );
+                                        bx.assume(cmp_start);
+                                    }
+
+                                    let enum_value_upper_bound =
                                         bx.cx().const_uint_big(ll_t_in, *scalar.valid_range.end());
-                                    let cmp = bx.icmp(IntPredicate::IntULE, llval, ll_t_in_const);
-                                    bx.assume(cmp);
+                                    let cmp_end = bx.icmp(
+                                        IntPredicate::IntULE,
+                                        llval,
+                                        enum_value_upper_bound,
+                                    );
+                                    bx.assume(cmp_end);
                                 }
                             }
                         }

src/test/codegen/enum-bounds-check-derived-idx.rs

@@ -0,0 +1,25 @@
+// This test checks an optimization that is not guaranteed to work. This test case should not block
+// a future LLVM update.
+// compile-flags: -O
+// min-llvm-version: 11.0
+
+#![crate_type = "lib"]
+
+pub enum Bar {
+    A = 1,
+    B = 3,
+}
+
+// CHECK-LABEL: @lookup_inc
+#[no_mangle]
+pub fn lookup_inc(buf: &[u8; 5], f: Bar) -> u8 {
+    // CHECK-NOT: panic_bounds_check
+    buf[f as usize + 1]
+}
+
+// CHECK-LABEL: @lookup_dec
+#[no_mangle]
+pub fn lookup_dec(buf: &[u8; 5], f: Bar) -> u8 {
+    // CHECK-NOT: panic_bounds_check
+    buf[f as usize - 1]
+}

src/test/codegen/enum-bounds-check-issue-13926.rs

@@ -0,0 +1,19 @@
+// This test checks an optimization that is not guaranteed to work. This test case should not block
+// a future LLVM update.
+// compile-flags: -O
+// min-llvm-version: 11.0
+
+#![crate_type = "lib"]
+
+#[repr(u8)]
+pub enum Exception {
+    Low = 5,
+    High = 10,
+}
+
+// CHECK-LABEL: @access
+#[no_mangle]
+pub fn access(array: &[usize; 12], exc: Exception) -> usize {
+    // CHECK-NOT: panic_bounds_check
+    array[(exc as u8 - 4) as usize]
+}

src/test/codegen/enum-bounds-check.rs

@@ -12,3 +12,15 @@ pub fn lookup(buf: &[u8; 2], f: Foo) -> u8 {
     // CHECK-NOT: panic_bounds_check
     buf[f as usize]
 }
+
+pub enum Bar {
+    A = 2,
+    B = 3
+}
+
+// CHECK-LABEL: @lookup_unmodified
+#[no_mangle]
+pub fn lookup_unmodified(buf: &[u8; 5], f: Bar) -> u8 {
+    // CHECK-NOT: panic_bounds_check
+    buf[f as usize]
+}