gvn: const promote arrays of any sizes

Author: tesuji

compiler/rustc_mir_transform/src/gvn.rs

@@ -366,12 +366,6 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
     #[instrument(level = "trace", skip(self), ret)]
     fn eval_to_const(&mut self, value: VnIndex) -> Option<OpTy<'tcx>> {
         use Value::*;
-        // LLVM optimizes the load of `sizeof(size_t) * 2` as a single `mov`,
-        // which is cheap. Bigger values make more `mov` instructions generated.
-        // After GVN, it became a single load (`lea`) of an address in `.rodata`.
-        // But to avoid blessing differences between 32-bit and 64-bit target,
-        // let's choose `size_t = u64`.
-        const STACK_THRESHOLD: u64 = std::mem::size_of::<u64>() as u64 * 2;
         let op = match *self.get(value) {
             Opaque(_) => return None,
             // Do not bother evaluating repeat expressions. This would uselessly consume memory.
@@ -423,9 +417,6 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     let ptr_imm = Immediate::new_pointer_with_meta(data, meta, &self.ecx);
                     ImmTy::from_immediate(ptr_imm, ty).into()
                 } else if matches!(kind, AggregateTy::Array) {
-                    if ty.layout.size().bytes() <= STACK_THRESHOLD {
-                        return None;
-                    }
                     let dest = self.ecx.allocate(ty, MemoryKind::Stack).ok()?;
                     // FIXME: Can we speed it up by using `ecx.write_immediate(.ScalarPair(_), dest)`?
                     for (field_index, op) in fields.iter().copied().enumerate() {

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

@@ -78,11 +78,14 @@
           StorageDead(_12);
           StorageLive(_13);
           StorageLive(_14);
-          _14 = [const 1_i32, const 0_i32, const 0_i32];
+-         _14 = [const 1_i32, const 0_i32, const 0_i32];
++         _14 = const [1_i32, 0_i32, 0_i32];
           StorageLive(_15);
-          _15 = [const 0_i32, const 1_i32, const 0_i32];
+-         _15 = [const 0_i32, const 1_i32, const 0_i32];
++         _15 = const [0_i32, 1_i32, 0_i32];
           StorageLive(_16);
-          _16 = [const 0_i32, const 0_i32, const 1_i32];
+-         _16 = [const 0_i32, const 0_i32, const 1_i32];
++         _16 = const [0_i32, 0_i32, 1_i32];
           _13 = [move _14, move _15, move _16];
           StorageDead(_16);
           StorageDead(_15);
@@ -96,35 +99,23 @@
           StorageDead(_1);
           return;
       }
-+ }
+  }
 + 
-+ ALLOC0 (size: 32, align: 4) {
-+     0x00 │ 00 00 80 3f 00 00 00 40 00 00 40 40 00 00 80 3f │ ...?...@..@@...?
-+     0x10 │ 00 00 80 3f 00 00 80 3f 00 00 80 3f 00 00 28 42 │ ...?...?...?..(B
-+ }
++ ALLOC0 (size: 12, align: 4) { .. }
 + 
-+ ALLOC1 (size: 20, align: 4) {
-+     0x00 │ 1f 00 00 00 60 00 00 00 ad 00 00 00 32 00 00 00 │ ....`.......2...
-+     0x10 │ 01 00 00 00                                     │ ....
-+ }
++ ALLOC1 (size: 12, align: 4) { .. }
 + 
-+ ALLOC2 (size: 20, align: 4) {
-+     0x00 │ c1 00 00 00 a4 00 00 00 c2 00 00 00 c5 00 00 00 │ ................
-+     0x10 │ 06 00 00 00                                     │ ....
-+ }
++ ALLOC2 (size: 12, align: 4) { .. }
 + 
-+ ALLOC3 (size: 20, align: 4) {
-+     0x00 │ b2 00 00 00 09 00 00 00 04 00 00 00 38 00 00 00 │ ............8...
-+     0x10 │ dd 00 00 00                                     │ ....
-+ }
++ ALLOC3 (size: 32, align: 4) { .. }
 + 
-+ 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                                     │ >...
-+ }
++ ALLOC4 (size: 20, align: 4) { .. }
 + 
-+ 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                                     │ >...
-  }
++ ALLOC5 (size: 20, align: 4) { .. }
++ 
++ ALLOC6 (size: 20, align: 4) { .. }
++ 
++ ALLOC7 (size: 20, align: 4) { .. }
++ 
++ ALLOC8 (size: 20, align: 4) { .. }
 

tests/mir-opt/const_array_locals.rs

@@ -1,4 +1,5 @@
 //@ test-mir-pass: GVN
+//@ compile-flags: -Zdump-mir-exclude-alloc-bytes
 #![feature(repr_simd)]
 
 #[repr(simd)]
@@ -33,9 +34,9 @@ pub fn main() {
     let _f = F32x8([1.0, 2.0, 3.0, 1.0, 1.0, 1.0, 1.0, 42.0]);
 
     // ice with small arrays
-    // CHECK: [[A:_[0-9]+]] = [const 1_i32, const 0_i32, const 0_i32];
-    // CHECK: [[B:_[0-9]+]] = [const 0_i32, const 1_i32, const 0_i32];
-    // CHECK: [[C:_[0-9]+]] = [const 0_i32, const 0_i32, const 1_i32];
+    // CHECK: [[A:_[0-9]+]] = const [1_i32, 0_i32, 0_i32];
+    // CHECK: [[B:_[0-9]+]] = const [0_i32, 1_i32, 0_i32];
+    // CHECK: [[C:_[0-9]+]] = const [0_i32, 0_i32, 1_i32];
     // CHECK: {{_[0-9]+}} = [move [[A]], move [[B]], move [[C]]];
     [[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
 }

tests/mir-opt/const_prop/array_index.main.GVN.32bit.panic-unwind.diff

@@ -15,7 +15,8 @@
       bb0: {
           StorageLive(_1);
           StorageLive(_2);
-          _2 = [const 0_u32, const 1_u32, const 2_u32, const 3_u32];
+-         _2 = [const 0_u32, const 1_u32, const 2_u32, const 3_u32];
++         _2 = const [0_u32, 1_u32, 2_u32, 3_u32];
           StorageLive(_3);
           _3 = const 2_usize;
 -         _4 = Len(_2);
@@ -36,4 +37,6 @@
           return;
       }
   }
++ 
++ ALLOC0 (size: 16, align: 4) { .. }
 

tests/mir-opt/const_prop/array_index.main.GVN.64bit.panic-unwind.diff

@@ -15,7 +15,8 @@
       bb0: {
           StorageLive(_1);
           StorageLive(_2);
-          _2 = [const 0_u32, const 1_u32, const 2_u32, const 3_u32];
+-         _2 = [const 0_u32, const 1_u32, const 2_u32, const 3_u32];
++         _2 = const [0_u32, 1_u32, 2_u32, 3_u32];
           StorageLive(_3);
           _3 = const 2_usize;
 -         _4 = Len(_2);
@@ -36,4 +37,6 @@
           return;
       }
   }
++ 
++ ALLOC0 (size: 16, align: 4) { .. }
 

tests/mir-opt/const_prop/array_index.rs

@@ -1,4 +1,5 @@
 //@ test-mir-pass: GVN
+//@ compile-flags: -Zdump-mir-exclude-alloc-bytes
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 // EMIT_MIR_FOR_EACH_BIT_WIDTH