Defer to llvm to deduplicate promote allocations

Between promote temps pass and gvn

Author: tesuji

compiler/rustc_mir_transform/src/gvn.rs

@@ -1488,15 +1488,6 @@ impl<'tcx> VnState<'_, 'tcx> {
 
         let op = self.evaluated[index].as_ref()?;
 
-        // Ignore promoted arrays. Promoted arrays are already placed in `.rodata`.
-        // Which is what we try to archive for running gvn on constant local arrays.
-        if let Either::Left(mplace) = op.as_mplace_or_imm()
-            && mplace.layout.ty.is_array()
-            && let Value::Projection(_index, ProjectionElem::Deref) = value
-        {
-            return None;
-        }
-
         let value = op_to_prop_const(&mut self.ecx, op)?;
 
         // Check that we do not leak a pointer.

tests/codegen/issue-73825-gvn-const-local-array.rs

@@ -6,7 +6,7 @@
 // CHECK-NEXT: start:
 // CHECK-NEXT: %_3 = and i64 %x, 63
 // CHECK-NEXT: %0 = getelementptr inbounds [64 x i32], ptr @0, i64 0, i64 %_3
-// CHECK-NEXT: %_0 = load i32, ptr %0, align 4, !noundef !3
+// CHECK-NEXT: %_0 = load i32, ptr %0, align 4
 // CHECK-NEXT: ret i32 %_0
 #[no_mangle]
 #[rustfmt::skip]
@@ -23,3 +23,29 @@ pub fn foo(x: usize) -> i32 {
     ];
     base[x % 64]
 }
+
+// This checks whether LLVM de-duplicates `promoted` array and `base` array.
+// Because in MIR, `&[..]` is already promoted by promote pass. GVN keeps promoting
+// `*&[..]` to `const [..]` again.
+//
+// CHECK-LABEL: @deduplicability
+// CHECK-NEXT: start:
+// CHECK-NEXT: %_3 = and i64 %x, 63
+// CHECK-NEXT: %0 = getelementptr inbounds [64 x i32], ptr @0, i64 0, i64 %_3
+// CHECK-NEXT: %_0 = load i32, ptr %0, align 4
+// CHECK-NEXT: ret i32 %_0
+#[no_mangle]
+#[rustfmt::skip]
+pub fn deduplicability(x: usize) -> i32 {
+    let promoted = *&[
+        67i32, 754, 860, 559, 368, 870, 548, 972,
+        141, 731, 351, 664, 32, 4, 996, 741,
+        203, 292, 237, 480, 151, 940, 777, 540,
+        143, 587, 747, 65, 152, 517, 882, 880,
+        712, 595, 370, 901, 237, 53, 789, 785,
+        912, 650, 896, 367, 316, 392, 62, 473,
+        675, 691, 281, 192, 445, 970, 225, 425,
+        628, 324, 322, 206, 912, 867, 462, 92
+    ];
+    promoted[x % 64]
+}

tests/mir-opt/const_array_locals.main.GVN.diff

@@ -59,7 +59,7 @@
           _17 = const main::promoted[0];
           _7 = &(*_17);
 -         _6 = (*_7);
-+         _6 = (*_17);
++         _6 = const [254_i32, 42_i32, 15_i32, 39_i32, 62_i32];
           StorageDead(_7);
           StorageLive(_9);
           StorageLive(_10);
@@ -122,6 +122,11 @@
 + 
 + ALLOC4 (size: 20, align: 4) {
 +     0x00 │ ff 00 00 00 69 00 00 00 0f 00 00 00 27 00 00 00 │ ....i.......'...
++     0x10 │ 3e 00 00 00                                     │ >...
++ }
++ 
++ ALLOC5 (size: 20, align: 4) {
++     0x00 │ fe 00 00 00 2a 00 00 00 0f 00 00 00 27 00 00 00 │ ....*.......'...
 +     0x10 │ 3e 00 00 00                                     │ >...
   }
 

tests/mir-opt/const_array_locals.rs

@@ -21,7 +21,7 @@ pub fn main() {
     // CHECK: [[_foo]] = [move [[subarray1]], move [[subarray2]]];
     let _foo = [[178, 9, 4, 56, 221], [193, 164, 194, 197, 6]];
     // CHECK: [[PROMOTED:_[0-9]+]] = const main::promoted[0];
-    // CHECK: [[_darr]] = (*[[PROMOTED]]);
+    // CHECK: [[_darr]] = const [254_i32, 42_i32, 15_i32, 39_i32, 62_i32];
     let _darr = *&[254, 42, 15, 39, 62];
 
     // CHECK: [[ARG:_[0-9]+]] = const [31_u32, 96_u32, 173_u32, 50_u32, 1_u32];