Also early exit from Rvalue::Use

Author: saethlin

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -26,6 +26,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     ) {
         match *rvalue {
             mir::Rvalue::Use(ref operand) => {
+                if let mir::Operand::Constant(const_op) = operand {
+                    let val = self.eval_mir_constant(const_op);
+                    if val.all_bytes_uninit(self.cx.tcx()) {
+                        return;
+                    }
+                }
+
                 let cg_operand = self.codegen_operand(bx, operand);
                 // FIXME: consider not copying constants through stack. (Fixable by codegen'ing
                 // constants into `OperandValue::Ref`; why don’t we do that yet if we don’t?)

tests/codegen/uninit-consts.rs

@@ -11,21 +11,18 @@ pub struct PartiallyUninit {
     y: MaybeUninit<[u8; 10]>,
 }
 
-// CHECK: [[FULLY_UNINIT:@.*]] = private unnamed_addr constant [10 x i8] undef
-
 // CHECK: [[PARTIALLY_UNINIT:@.*]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"{{\\EF\\BE\\AD\\DE|\\DE\\AD\\BE\\EF}}", [12 x i8] undef }>, align 4
 
 // This shouldn't contain undef, since it contains more chunks
 // than the default value of uninit_const_chunk_threshold.
 // CHECK: [[UNINIT_PADDING_HUGE:@.*]] = private unnamed_addr constant [32768 x i8] c"{{.+}}", align 4
 
-// CHECK: [[FULLY_UNINIT_HUGE:@.*]] = private unnamed_addr constant [16384 x i8] undef
-
 // CHECK-LABEL: @fully_uninit
 #[no_mangle]
 pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
     const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
-    // CHECK: call void @llvm.memcpy.{{.+}}(ptr align 1 %_0, ptr align 1 {{.*}}[[FULLY_UNINIT]]{{.*}}, i{{(32|64)}} 10, i1 false)
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: ret void
     M
 }
 
@@ -49,6 +46,7 @@ pub const fn uninit_padding_huge() -> [(u32, u8); 4096] {
 #[no_mangle]
 pub const fn fully_uninit_huge() -> MaybeUninit<[u32; 4096]> {
     const F: MaybeUninit<[u32; 4096]> = MaybeUninit::uninit();
-    // CHECK: call void @llvm.memcpy.{{.+}}(ptr align 4 %_0, ptr align 4 {{.*}}[[FULLY_UNINIT_HUGE]]{{.*}}, i{{(32|64)}} 16384, i1 false)
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: ret void
     F
 }