Auto merge of rust-lang#134117 - DianQK:gep-i8, r=oli-obk

Modifies the index instruction from `gep [0 x %Type]` to `gep %Type`

Fixes rust-lang#133979.

This PR modifies the index instruction from `gep [0 x %Type]` to `gep %Type`, which is the same with pointer offset calculation.

This will help LLVM calculate various formats of GEP instructions. According to [[RFC] Replacing getelementptr with ptradd](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699), we ultimately aim to canonicalize everything to `gep i8`. Based on the results from rust-lang#134117 (comment), I think we still need to investigate some missing optimizations, so this PR is just a small step forward.

r? compiler

Author: bors

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -422,10 +422,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
             layout.size
         };
 
-        let llval = bx.inbounds_gep(bx.cx().backend_type(self.layout), self.val.llval, &[
-            bx.cx().const_usize(0),
-            llindex,
-        ]);
+        let llval = bx.inbounds_gep(bx.cx().backend_type(layout), self.val.llval, &[llindex]);
         let align = self.val.align.restrict_for_offset(offset);
         PlaceValue::new_sized(llval, align).with_type(layout)
     }

tests/codegen/bounds-checking/gep-issue-133979.rs

@@ -0,0 +1,22 @@
+//! Issue: <https://github.com/rust-lang/rust/issues/133979>
+//! Check that bounds checking are eliminated.
+
+//@ compile-flags: -Copt-level=2
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @test(
+#[no_mangle]
+fn test(a: &[&[u8]]) -> u32 {
+    // CHECK-NOT: panic_bounds_check
+    a.iter()
+        .enumerate()
+        .map(|(y, b)| {
+            b.iter()
+                .enumerate()
+                .filter(|(_, c)| **c == b'A')
+                .map(|(x, _)| a[y][x] as u32)
+                .sum::<u32>()
+        })
+        .sum()
+}

tests/codegen/gep-index.rs

@@ -0,0 +1,37 @@
+//! Check that index and offset use the same getelementptr format.
+
+//@ revisions: NO-OPT OPT
+//@[NO-OPT] compile-flags: -Copt-level=0
+//@[OPT] compile-flags: -Copt-level=1
+
+#![crate_type = "lib"]
+
+struct Foo(i32, i32);
+
+// CHECK-LABEL: @index_on_struct(
+#[no_mangle]
+fn index_on_struct(a: &[Foo], index: usize) -> &Foo {
+    // CHECK: getelementptr inbounds %Foo, ptr %a.0, {{i64|i32}} %index
+    &a[index]
+}
+
+// CHECK-LABEL: @offset_on_struct(
+#[no_mangle]
+fn offset_on_struct(a: *const Foo, index: usize) -> *const Foo {
+    // CHECK: getelementptr inbounds %Foo, ptr %a, {{i64|i32}} %index
+    unsafe { a.add(index) }
+}
+
+// CHECK-LABEL: @index_on_i32(
+#[no_mangle]
+fn index_on_i32(a: &[i32], index: usize) -> &i32 {
+    // CHECK: getelementptr inbounds i32, ptr %a.0, {{i64|i32}} %index
+    &a[index]
+}
+
+// CHECK-LABEL: @offset_on_i32(
+#[no_mangle]
+fn offset_on_i32(a: *const i32, index: usize) -> *const i32 {
+    // CHECK: getelementptr inbounds i32, ptr %a, {{i64|i32}} %index
+    unsafe { a.add(index) }
+}