[codegen] assume the tag, not the relative discriminant

scottmcm · scottmcm · commit b90e0fb67c2e · 2025-08-01T11:00:55.000-07:00
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -498,6 +498,35 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                         bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64);
                     (is_niche, tagged_discr, 0)
                 } else {
+                    // Thanks to parameter attributes and load metadata, LLVM already knows
+                    // the general valid range of the tag. It's possible, though, for there
+                    // to be an impossible value *in the middle*, which those ranges don't
+                    // communicate, so it's worth an `assume` to let the optimizer know.
+                    // Most importantly, this means when optimizing a variant test like
+                    // `SELECT(is_niche, complex, CONST) == CONST` it's ok to simplify that
+                    // to `!is_niche` because the `complex` part can't possibly match.
+                    //
+                    // This was previously asserted on `tagged_discr` below, where the
+                    // impossible value is more obvious, but that caused an intermediate
+                    // value to become multi-use and thus not optimize, so instead this
+                    // assumes on the original input which is always multi-use. See
+                    // <https://github.com/llvm/llvm-project/issues/134024#issuecomment-3131782555>
+                    //
+                    // FIXME: If we ever get range assume operand bundles in LLVM (so we
+                    // don't need the `icmp`s in the instruction stream any more), it
+                    // might be worth moving this back to being on the switch argument
+                    // where it's more obviously applicable.
+                    if niche_variants.contains(&untagged_variant)
+                        && bx.cx().sess().opts.optimize != OptLevel::No
+                    {
+                        let impossible = niche_start
+                            .wrapping_add(u128::from(untagged_variant.as_u32()))
+                            .wrapping_sub(u128::from(niche_variants.start().as_u32()));
+                        let impossible = bx.cx().const_uint_big(tag_llty, impossible);
+                        let ne = bx.icmp(IntPredicate::IntNE, tag, impossible);
+                        bx.assume(ne);
+                    }
+
                     // With multiple niched variants we'll have to actually compute
                     // the variant index from the stored tag.
                     //
@@ -588,20 +617,6 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let untagged_variant_const =
                     bx.cx().const_uint(cast_to, u64::from(untagged_variant.as_u32()));
 
-                // Thanks to parameter attributes and load metadata, LLVM already knows
-                // the general valid range of the tag. It's possible, though, for there
-                // to be an impossible value *in the middle*, which those ranges don't
-                // communicate, so it's worth an `assume` to let the optimizer know.
-                // Most importantly, this means when optimizing a variant test like
-                // `SELECT(is_niche, complex, CONST) == CONST` it's ok to simplify that
-                // to `!is_niche` because the `complex` part can't possibly match.
-                if niche_variants.contains(&untagged_variant)
-                    && bx.cx().sess().opts.optimize != OptLevel::No
-                {
-                    let ne = bx.icmp(IntPredicate::IntNE, tagged_discr, untagged_variant_const);
-                    bx.assume(ne);
-                }
-
                 let discr = bx.select(is_niche, tagged_discr, untagged_variant_const);
 
                 // In principle we could insert assumes on the possible range of `discr`, but
diff --git a/tests/codegen-llvm/enum/enum-discriminant-eq.rs b/tests/codegen-llvm/enum/enum-discriminant-eq.rs
@@ -91,16 +91,16 @@ pub enum Mid<T> {
 pub fn mid_bool_eq_discr(a: Mid<bool>, b: Mid<bool>) -> bool {
     // CHECK-LABEL: @mid_bool_eq_discr(
 
+    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i8 %a, 3
+    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
     // CHECK: %[[A_REL_DISCR:.+]] = add nsw i8 %a, -2
     // CHECK: %[[A_IS_NICHE:.+]] = icmp samesign ugt i8 %a, 1
-    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i8 %[[A_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
     // CHECK: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i8 %[[A_REL_DISCR]], i8 1
 
+    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i8 %b, 3
+    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
     // CHECK: %[[B_REL_DISCR:.+]] = add nsw i8 %b, -2
     // CHECK: %[[B_IS_NICHE:.+]] = icmp samesign ugt i8 %b, 1
-    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i8 %[[B_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
     // CHECK: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i8 %[[B_REL_DISCR]], i8 1
 
     // CHECK: ret i1 %[[R]]
@@ -111,16 +111,16 @@ pub fn mid_bool_eq_discr(a: Mid<bool>, b: Mid<bool>) -> bool {
 pub fn mid_ord_eq_discr(a: Mid<Ordering>, b: Mid<Ordering>) -> bool {
     // CHECK-LABEL: @mid_ord_eq_discr(
 
+    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i8 %a, 3
+    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
     // CHECK: %[[A_REL_DISCR:.+]] = add nsw i8 %a, -2
     // CHECK: %[[A_IS_NICHE:.+]] = icmp sgt i8 %a, 1
-    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i8 %[[A_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
     // CHECK: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i8 %[[A_REL_DISCR]], i8 1
 
+    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i8 %b, 3
+    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
     // CHECK: %[[B_REL_DISCR:.+]] = add nsw i8 %b, -2
     // CHECK: %[[B_IS_NICHE:.+]] = icmp sgt i8 %b, 1
-    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i8 %[[B_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
     // CHECK: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i8 %[[B_REL_DISCR]], i8 1
 
     // CHECK: %[[R:.+]] = icmp eq i8 %[[A_DISCR]], %[[B_DISCR]]
@@ -140,16 +140,16 @@ pub fn mid_nz32_eq_discr(a: Mid<NonZero<u32>>, b: Mid<NonZero<u32>>) -> bool {
 pub fn mid_ac_eq_discr(a: Mid<AC>, b: Mid<AC>) -> bool {
     // CHECK-LABEL: @mid_ac_eq_discr(
 
-    // LLVM20: %[[A_REL_DISCR:.+]] = xor i8 %a, -128
-    // CHECK: %[[A_IS_NICHE:.+]] = icmp slt i8 %a, 0
     // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i8 %a, -127
     // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
+    // LLVM20: %[[A_REL_DISCR:.+]] = xor i8 %a, -128
+    // CHECK: %[[A_IS_NICHE:.+]] = icmp slt i8 %a, 0
     // LLVM20: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i8 %[[A_REL_DISCR]], i8 1
 
-    // LLVM20: %[[B_REL_DISCR:.+]] = xor i8 %b, -128
-    // CHECK: %[[B_IS_NICHE:.+]] = icmp slt i8 %b, 0
     // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i8 %b, -127
     // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
+    // LLVM20: %[[B_REL_DISCR:.+]] = xor i8 %b, -128
+    // CHECK: %[[B_IS_NICHE:.+]] = icmp slt i8 %b, 0
     // LLVM20: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i8 %[[B_REL_DISCR]], i8 1
 
     // LLVM21: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i8 %a, i8 -127
@@ -166,21 +166,25 @@ pub fn mid_ac_eq_discr(a: Mid<AC>, b: Mid<AC>) -> bool {
 pub fn mid_giant_eq_discr(a: Mid<Giant>, b: Mid<Giant>) -> bool {
     // CHECK-LABEL: @mid_giant_eq_discr(
 
+    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i128 %a, 6
+    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
     // CHECK: %[[A_TRUNC:.+]] = trunc nuw nsw i128 %a to i64
-    // CHECK: %[[A_REL_DISCR:.+]] = add nsw i64 %[[A_TRUNC]], -5
+    // LLVM20: %[[A_REL_DISCR:.+]] = add nsw i64 %[[A_TRUNC]], -5
     // CHECK: %[[A_IS_NICHE:.+]] = icmp samesign ugt i128 %a, 4
-    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i64 %[[A_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[A_NOT_HOLE]])
-    // CHECK: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_REL_DISCR]], i64 1
+    // LLVM20: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_REL_DISCR]], i64 1
 
+    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i128 %b, 6
+    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
     // CHECK: %[[B_TRUNC:.+]] = trunc nuw nsw i128 %b to i64
-    // CHECK: %[[B_REL_DISCR:.+]] = add nsw i64 %[[B_TRUNC]], -5
+    // LLVM20: %[[B_REL_DISCR:.+]] = add nsw i64 %[[B_TRUNC]], -5
     // CHECK: %[[B_IS_NICHE:.+]] = icmp samesign ugt i128 %b, 4
-    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i64 %[[B_REL_DISCR]], 1
-    // CHECK: tail call void @llvm.assume(i1 %[[B_NOT_HOLE]])
-    // CHECK: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i64 %[[B_REL_DISCR]], i64 1
+    // LLVM20: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i64 %[[B_REL_DISCR]], i64 1
+
+    // LLVM21: %[[A_MODIFIED_TAG:.+]] = select i1 %[[A_IS_NICHE]], i64 %[[A_TRUNC]], i64 6
+    // LLVM21: %[[B_MODIFIED_TAG:.+]] = select i1 %[[B_IS_NICHE]], i64 %[[B_TRUNC]], i64 6
+    // LLVM21: %[[R:.+]] = icmp eq i64 %[[A_MODIFIED_TAG]], %[[B_MODIFIED_TAG]]
 
-    // CHECK: %[[R:.+]] = icmp eq i64 %[[A_DISCR]], %[[B_DISCR]]
+    // LLVM20: %[[R:.+]] = icmp eq i64 %[[A_DISCR]], %[[B_DISCR]]
     // CHECK: ret i1 %[[R]]
     discriminant_value(&a) == discriminant_value(&b)
 }
diff --git a/tests/codegen-llvm/enum/enum-match.rs b/tests/codegen-llvm/enum/enum-match.rs
@@ -138,18 +138,18 @@ pub fn match3(e: Option<&u8>) -> i16 {
 
 #[derive(PartialEq)]
 pub enum MiddleNiche {
-    A,
-    B,
-    C(bool),
-    D,
-    E,
+    A,       // tag 2
+    B,       // tag 3
+    C(bool), // untagged
+    D,       // tag 5
+    E,       // tag 6
 }
 
 // CHECK-LABEL: define{{( dso_local)?}} noundef{{( range\(i8 -?[0-9]+, -?[0-9]+\))?}} i8 @match4(i8{{.+}}%0)
 // CHECK-NEXT: start:
-// CHECK-NEXT: %[[REL_VAR:.+]] = add{{( nsw)?}} i8 %0, -2
-// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %[[REL_VAR]], 2
+// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %0, 4
 // CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
+// CHECK-NEXT: %[[REL_VAR:.+]] = add{{( nsw)?}} i8 %0, -2
 // CHECK-NEXT: %[[NOT_NICHE:.+]] = icmp{{( samesign)?}} ult i8 %0, 2
 // CHECK-NEXT: %[[DISCR:.+]] = select i1 %[[NOT_NICHE]], i8 2, i8 %[[REL_VAR]]
 // CHECK-NEXT: switch i8 %[[DISCR]]
@@ -443,19 +443,19 @@ pub enum HugeVariantIndex {
     V255(Never),
     V256(Never),
 
-    Possible257,
-    Bool258(bool),
-    Possible259,
+    Possible257,   // tag 2
+    Bool258(bool), // untagged
+    Possible259,   // tag 4
 }
 
 // CHECK-LABEL: define{{( dso_local)?}} noundef{{( range\(i8 [0-9]+, [0-9]+\))?}} i8 @match5(i8{{.+}}%0)
 // CHECK-NEXT: start:
+// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %0, 3
+// CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
 // CHECK-NEXT: %[[REL_VAR:.+]] = add{{( nsw)?}} i8 %0, -2
 // CHECK-NEXT: %[[REL_VAR_WIDE:.+]] = zext i8 %[[REL_VAR]] to i64
 // CHECK-NEXT: %[[IS_NICHE:.+]] = icmp{{( samesign)?}} ugt i8 %0, 1
 // CHECK-NEXT: %[[NICHE_DISCR:.+]] = add nuw nsw i64 %[[REL_VAR_WIDE]], 257
-// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i64 %[[NICHE_DISCR]], 258
-// CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
 // CHECK-NEXT: %[[DISCR:.+]] = select i1 %[[IS_NICHE]], i64 %[[NICHE_DISCR]], i64 258
 // CHECK-NEXT: switch i64 %[[DISCR]],
 // CHECK-NEXT:   i64 257,
