Auto merge of #138391 - scottmcm:SSA-discriminants, r=WaffleLapkin

Don't `alloca` just to look at a discriminant

Today we're making LLVM do a bunch of extra work when you match on trivial stuff like `Option<bool>` or `ControlFlow<u8>`.

This PR changes that so that simple types like `Option<u32>` or `Result<(), Box<Error>>` can stay as `OperandValue::ScalarPair` and we can still read the discriminant from them, rather than needing to write them into memory to have a `PlaceValue` just to get the discriminant out.

Fixes #137503

1 files changed, 20 insertions, 17 deletions

diff --git a/tests/codegen/enum/enum-two-variants-match.rs b/tests/codegen/enum/enum-two-variants-match.rs
index e5978bfc761..c1f208d7909 100644
--- a/tests/codegen/enum/enum-two-variants-match.rs
+++ b/tests/codegen/enum/enum-two-variants-match.rs
@@ -7,19 +7,22 @@
 // CHECK-LABEL: @option_match
 #[no_mangle]
 pub fn option_match(x: Option<i32>) -> u16 {
-    // CHECK: %x = alloca [8 x i8]
-    // CHECK: store i32 %0, ptr %x
-    // CHECK: %[[TAG:.+]] = load i32, ptr %x
-    // CHECK-SAME: !range ![[ZERO_ONE_32:[0-9]+]]
-    // CHECK: %[[DISCR:.+]] = zext i32 %[[TAG]] to i64
+    // CHECK-NOT: %x = alloca
+    // CHECK: %[[OUT:.+]] = alloca [2 x i8]
+    // CHECK-NOT: %x = alloca
+
+    // CHECK: %[[DISCR:.+]] = zext i32 %x.0 to i64
     // CHECK: %[[COND:.+]] = trunc nuw i64 %[[DISCR]] to i1
     // CHECK: br i1 %[[COND]], label %[[TRUE:[a-z0-9]+]], label %[[FALSE:[a-z0-9]+]]
 
     // CHECK: [[TRUE]]:
-    // CHECK: store i16 13
+    // CHECK: store i16 13, ptr %[[OUT]]
 
     // CHECK: [[FALSE]]:
-    // CHECK: store i16 42
+    // CHECK: store i16 42, ptr %[[OUT]]
+
+    // CHECK: %[[RET:.+]] = load i16, ptr %[[OUT]]
+    // CHECK: ret i16 %[[RET]]
     match x {
         Some(_) => 13,
         None => 42,
@@ -29,23 +32,23 @@ pub fn option_match(x: Option<i32>) -> u16 {
 // CHECK-LABEL: @result_match
 #[no_mangle]
 pub fn result_match(x: Result<u64, i64>) -> u16 {
-    // CHECK: %x = alloca [16 x i8]
-    // CHECK: store i64 %0, ptr %x
-    // CHECK: %[[DISCR:.+]] = load i64, ptr %x
-    // CHECK-SAME: !range ![[ZERO_ONE_64:[0-9]+]]
-    // CHECK: %[[COND:.+]] = trunc nuw i64 %[[DISCR]] to i1
+    // CHECK-NOT: %x = alloca
+    // CHECK: %[[OUT:.+]] = alloca [2 x i8]
+    // CHECK-NOT: %x = alloca
+
+    // CHECK: %[[COND:.+]] = trunc nuw i64 %x.0 to i1
     // CHECK: br i1 %[[COND]], label %[[TRUE:[a-z0-9]+]], label %[[FALSE:[a-z0-9]+]]
 
     // CHECK: [[TRUE]]:
-    // CHECK: store i16 13
+    // CHECK: store i16 13, ptr %[[OUT]]
 
     // CHECK: [[FALSE]]:
-    // CHECK: store i16 42
+    // CHECK: store i16 42, ptr %[[OUT]]
+
+    // CHECK: %[[RET:.+]] = load i16, ptr %[[OUT]]
+    // CHECK: ret i16 %[[RET]]
     match x {
         Err(_) => 13,
         Ok(_) => 42,
     }
 }
-
-// CHECK: ![[ZERO_ONE_32]] = !{i32 0, i32 2}
-// CHECK: ![[ZERO_ONE_64]] = !{i64 0, i64 2}