From c88b167f1518370833c9216706d09735d1e2d697 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C3=A1niel=20Buga?= <bugadani@gmail.com>
Date: Sun, 30 Aug 2020 21:26:03 +0200
Subject: [PATCH 1/2] Additional assumption to elide bounds check

---
 compiler/rustc_codegen_ssa/src/mir/rvalue.rs | 26 ++++++++++++++++----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 71f924df119a3..43521048012fa 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/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);
                                 }
                             }
                         }

From 1d157ce797dddcee16a577796199b1144b4f7f34 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C3=A1niel=20Buga?= <bugadani@gmail.com>
Date: Tue, 25 Aug 2020 11:44:18 +0200
Subject: [PATCH 2/2] Add codegen tests

---
 .../codegen/enum-bounds-check-derived-idx.rs  | 25 +++++++++++++++++++
 .../codegen/enum-bounds-check-issue-13926.rs  | 19 ++++++++++++++
 src/test/codegen/enum-bounds-check.rs         | 12 +++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 src/test/codegen/enum-bounds-check-derived-idx.rs
 create mode 100644 src/test/codegen/enum-bounds-check-issue-13926.rs

diff --git a/src/test/codegen/enum-bounds-check-derived-idx.rs b/src/test/codegen/enum-bounds-check-derived-idx.rs
new file mode 100644
index 0000000000000..7e3773b6a3eb1
--- /dev/null
+++ b/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]
+}
diff --git a/src/test/codegen/enum-bounds-check-issue-13926.rs b/src/test/codegen/enum-bounds-check-issue-13926.rs
new file mode 100644
index 0000000000000..ad029f0fa7399
--- /dev/null
+++ b/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]
+}
diff --git a/src/test/codegen/enum-bounds-check.rs b/src/test/codegen/enum-bounds-check.rs
index 21a27c9f35d4e..17322d5911b92 100644
--- a/src/test/codegen/enum-bounds-check.rs
+++ b/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]
+}