Auto merge of #129283 - saethlin:unreachable-allocas, r=scottmcm

Don't alloca for unused locals

We already have a concept of mono-unreachable basic blocks; this is primarily useful for ensuring that we do not compile code under an `if false`. But since we never gave locals the same analysis, a large local only used under an `if false` will still have stack space allocated for it.

There are 3 places we traverse MIR during monomorphization: Inside the collector, `non_ssa_locals`, and the walk to generate code. Unfortunately, https://github.com/rust-lang/rust/pull/129283#issuecomment-2297925578 indicates that we cannot afford the expense of tracking reachable locals during the collector's traversal, so we do need at least two mono-reachable traversals. And of course caching is of no help here because the benchmarks that regress are incr-unchanged; they don't do any codegen.

This fixes the second problem in https://github.com/rust-lang/rust/issues/129282, and brings us anther step toward `const if` at home.

2 files changed, 33 insertions, 8 deletions

diff --git a/tests/codegen/constant-branch.rs b/tests/codegen/constant-branch.rs
index a2710cc4b25..8fc8fb4f57a 100644
--- a/tests/codegen/constant-branch.rs
+++ b/tests/codegen/constant-branch.rs
@@ -7,18 +7,19 @@
 // CHECK-LABEL: @if_bool
 #[no_mangle]
 pub fn if_bool() {
-    // CHECK: br label %{{.+}}
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     _ = if true { 0 } else { 1 };
 
-    // CHECK: br label %{{.+}}
     _ = if false { 0 } else { 1 };
 }
 
 // CHECK-LABEL: @if_constant_int_eq
 #[no_mangle]
 pub fn if_constant_int_eq() {
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     let val = 0;
-    // CHECK: br label %{{.+}}
     _ = if val == 0 { 0 } else { 1 };
 
     // CHECK: br label %{{.+}}
@@ -28,23 +29,20 @@ pub fn if_constant_int_eq() {
 // CHECK-LABEL: @if_constant_match
 #[no_mangle]
 pub fn if_constant_match() {
-    // CHECK: br label %{{.+}}
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     _ = match 1 {
         1 => 2,
         2 => 3,
         _ => 4,
     };
 
-    // CHECK: br label %{{.+}}
     _ = match 1 {
         2 => 3,
         _ => 4,
     };
 
-    // CHECK: br label %[[MINUS1:.+]]
     _ = match -1 {
-        // CHECK: [[MINUS1]]:
-        // CHECK: store i32 1
         -1 => 1,
         _ => 0,
     }
diff --git a/tests/codegen/no-alloca-inside-if-false.rs b/tests/codegen/no-alloca-inside-if-false.rs
new file mode 100644
index 00000000000..a231c7e808a
--- /dev/null
+++ b/tests/codegen/no-alloca-inside-if-false.rs
@@ -0,0 +1,27 @@
+//@ compile-flags: -Cno-prepopulate-passes -Copt-level=0 -Cpanic=abort
+// Check that there's an alloca for the reference and the vector, but nothing else.
+// We use panic=abort because unwinding panics give hint::black_box a cleanup block, which has
+// another alloca.
+
+#![crate_type = "lib"]
+
+#[inline(never)]
+fn test<const SIZE: usize>() {
+    // CHECK-LABEL: no_alloca_inside_if_false::test
+    // CHECK: start:
+    // CHECK-NEXT: alloca [{{12|24}} x i8]
+    // CHECK-NOT: alloca
+    if const { SIZE < 4096 } {
+        let arr = [0u8; SIZE];
+        std::hint::black_box(&arr);
+    } else {
+        let vec = vec![0u8; SIZE];
+        std::hint::black_box(&vec);
+    }
+}
+
+// CHECK-LABEL: @main
+#[no_mangle]
+pub fn main() {
+    test::<8192>();
+}