use non-inbounds GEP for ZSTs, add fixmes

Author: erikdesjardins

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -104,6 +104,10 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         let mut simple = || {
             let llval = if offset.bytes() == 0 {
                 self.llval
+            } else if field.is_zst() {
+                // FIXME(erikdesjardins): it should be fine to use inbounds for ZSTs too;
+                // keeping this logic for now to preserve previous behavior.
+                bx.ptradd(self.llval, bx.const_usize(offset.bytes()))
             } else {
                 bx.inbounds_ptradd(self.llval, bx.const_usize(offset.bytes()))
             };
@@ -164,6 +168,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         debug!("struct_field_ptr: DST field offset: {:?}", offset);
 
         // Adjust pointer.
+        // FIXME(erikdesjardins): should be able to use inbounds here too.
         let ptr = bx.ptradd(self.llval, offset);
 
         PlaceRef { llval: ptr, llextra: self.llextra, layout: field, align: effective_field_align }

tests/codegen/zst-offset.rs

@@ -13,7 +13,7 @@ pub fn helper(_: usize) {
 // CHECK-LABEL: @scalar_layout
 #[no_mangle]
 pub fn scalar_layout(s: &(u64, ())) {
-// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 8
+// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 8
     let x = &s.1;
     witness(&x); // keep variable in an alloca
 }
@@ -22,7 +22,7 @@ pub fn scalar_layout(s: &(u64, ())) {
 // CHECK-LABEL: @scalarpair_layout
 #[no_mangle]
 pub fn scalarpair_layout(s: &(u64, u32, ())) {
-// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 12
+// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 12
     let x = &s.2;
     witness(&x); // keep variable in an alloca
 }
@@ -34,7 +34,7 @@ pub struct U64x4(u64, u64, u64, u64);
 // CHECK-LABEL: @vector_layout
 #[no_mangle]
 pub fn vector_layout(s: &(U64x4, ())) {
-// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 32
+// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 32
     let x = &s.1;
     witness(&x); // keep variable in an alloca
 }