Simplify codegen for niche-encoded variant tests

scottmcm · scottmcm · commit d5bcfb334b61 · 2025-07-12T04:53:24.000-07:00
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
@@ -43,7 +43,7 @@ use std::fmt;
 #[cfg(feature = "nightly")]
 use std::iter::Step;
 use std::num::{NonZeroUsize, ParseIntError};
-use std::ops::{Add, AddAssign, Deref, Mul, RangeInclusive, Sub};
+use std::ops::{Add, AddAssign, Deref, Mul, RangeFull, RangeInclusive, Sub};
 use std::str::FromStr;
 
 use bitflags::bitflags;
@@ -1391,12 +1391,45 @@ impl WrappingRange {
     }
 
     /// Returns `true` if `size` completely fills the range.
+    ///
+    /// Note that this is *not* the same as `self == WrappingRange::full(size)`.
+    /// Niche calculations can produce full ranges which are not the canonical one;
+    /// for example `Option<NonZero<u16>>` gets `valid_range: (..=0) | (1..)`.
     #[inline]
     fn is_full_for(&self, size: Size) -> bool {
         let max_value = size.unsigned_int_max();
         debug_assert!(self.start <= max_value && self.end <= max_value);
         self.start == (self.end.wrapping_add(1) & max_value)
     }
+
+    /// Checks whether this range is considered non-wrapping when the values are
+    /// interpreted as *unsigned* numbers of width `size`.
+    ///
+    /// Returns `Ok(true)` if there's no wrap-around, `Ok(false)` if there is,
+    /// and `Err(..)` if the range is full so it depends how you think about it.
+    #[inline]
+    pub fn no_unsigned_wraparound(&self, size: Size) -> Result<bool, RangeFull> {
+        if self.is_full_for(size) { Err(..) } else { Ok(self.start <= self.end) }
+    }
+
+    /// Checks whether this range is considered non-wrapping when the values are
+    /// interpreted as *signed* numbers of width `size`.
+    ///
+    /// This is heavily dependent on the `size`, as `100..=200` does wrap when
+    /// interpreted as `i8`, but doesn't when interpreted as `i16`.
+    ///
+    /// Returns `Ok(true)` if there's no wrap-around, `Ok(false)` if there is,
+    /// and `Err(..)` if the range is full so it depends how you think about it.
+    #[inline]
+    pub fn no_signed_wraparound(&self, size: Size) -> Result<bool, RangeFull> {
+        if self.is_full_for(size) {
+            Err(..)
+        } else {
+            let start: i128 = size.sign_extend(self.start);
+            let end: i128 = size.sign_extend(self.end);
+            Ok(start <= end)
+        }
+    }
 }
 
 impl fmt::Debug for WrappingRange {
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -486,6 +486,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 // value and the variant index match, since that's all `Niche` can encode.
 
                 let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
+                let niche_start_const = bx.cx().const_uint_big(tag_llty, niche_start);
 
                 // We have a subrange `niche_start..=niche_end` inside `range`.
                 // If the value of the tag is inside this subrange, it's a
@@ -511,35 +512,44 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                     // } else {
                     //     untagged_variant
                     // }
-                    let niche_start = bx.cx().const_uint_big(tag_llty, niche_start);
-                    let is_niche = bx.icmp(IntPredicate::IntEQ, tag, niche_start);
+                    let is_niche = bx.icmp(IntPredicate::IntEQ, tag, niche_start_const);
                     let tagged_discr =
                         bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64);
                     (is_niche, tagged_discr, 0)
                 } else {
                     // The special cases don't apply, so we'll have to go with
                     // the general algorithm.
-                    let relative_discr = bx.sub(tag, bx.cx().const_uint_big(tag_llty, niche_start));
+
+                    let tag_range = tag_scalar.valid_range(&dl);
+                    let tag_size = tag_scalar.size(&dl);
+                    let niche_end = u128::from(relative_max).wrapping_add(niche_start);
+                    let niche_end = tag_size.truncate(niche_end);
+
+                    let relative_discr = bx.sub(tag, niche_start_const);
                     let cast_tag = bx.intcast(relative_discr, cast_to, false);
-                    let is_niche = bx.icmp(
-                        IntPredicate::IntULE,
-                        relative_discr,
-                        bx.cx().const_uint(tag_llty, relative_max as u64),
-                    );
-
-                    // 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.
-                    if niche_variants.contains(&untagged_variant)
-                        && bx.cx().sess().opts.optimize != OptLevel::No
-                    {
-                        let impossible =
-                            u64::from(untagged_variant.as_u32() - niche_variants.start().as_u32());
-                        let impossible = bx.cx().const_uint(tag_llty, impossible);
-                        let ne = bx.icmp(IntPredicate::IntNE, relative_discr, impossible);
-                        bx.assume(ne);
-                    }
+                    let is_niche = if tag_range.no_unsigned_wraparound(tag_size) == Ok(true) {
+                        if niche_start == tag_range.start {
+                            let niche_end_const = bx.cx().const_uint_big(tag_llty, niche_end);
+                            bx.icmp(IntPredicate::IntULE, tag, niche_end_const)
+                        } else {
+                            assert_eq!(niche_end, tag_range.end);
+                            bx.icmp(IntPredicate::IntUGE, tag, niche_start_const)
+                        }
+                    } else if tag_range.no_signed_wraparound(tag_size) == Ok(true) {
+                        if niche_start == tag_range.start {
+                            let niche_end_const = bx.cx().const_uint_big(tag_llty, niche_end);
+                            bx.icmp(IntPredicate::IntSLE, tag, niche_end_const)
+                        } else {
+                            assert_eq!(niche_end, tag_range.end);
+                            bx.icmp(IntPredicate::IntSGE, tag, niche_start_const)
+                        }
+                    } else {
+                        bx.icmp(
+                            IntPredicate::IntULE,
+                            relative_discr,
+                            bx.cx().const_uint(tag_llty, relative_max as u64),
+                        )
+                    };
 
                     (is_niche, cast_tag, niche_variants.start().as_u32() as u128)
                 };
@@ -550,11 +560,24 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                     bx.add(tagged_discr, bx.cx().const_uint_big(cast_to, delta))
                 };
 
-                let discr = bx.select(
-                    is_niche,
-                    tagged_discr,
-                    bx.cx().const_uint(cast_to, untagged_variant.as_u32() as u64),
-                );
+                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
                 // currently in LLVM this isn't worth it because the original `tag` will
diff --git a/tests/codegen/enum/enum-discriminant-eq.rs b/tests/codegen/enum/enum-discriminant-eq.rs
@@ -89,13 +89,13 @@ pub fn mid_bool_eq_discr(a: Mid<bool>, b: Mid<bool>) -> bool {
     // CHECK-LABEL: @mid_bool_eq_discr(
 
     // CHECK: %[[A_REL_DISCR:.+]] = add nsw i8 %a, -2
-    // CHECK: %[[A_IS_NICHE:.+]] = icmp ult i8 %[[A_REL_DISCR]], 3
+    // 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_REL_DISCR:.+]] = add nsw i8 %b, -2
-    // CHECK: %[[B_IS_NICHE:.+]] = icmp ult i8 %[[B_REL_DISCR]], 3
+    // 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
@@ -109,13 +109,13 @@ pub fn mid_ord_eq_discr(a: Mid<Ordering>, b: Mid<Ordering>) -> bool {
     // CHECK-LABEL: @mid_ord_eq_discr(
 
     // CHECK: %[[A_REL_DISCR:.+]] = add nsw i8 %a, -2
-    // CHECK: %[[A_IS_NICHE:.+]] = icmp ult i8 %[[A_REL_DISCR]], 3
+    // 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_REL_DISCR:.+]] = add nsw i8 %b, -2
-    // CHECK: %[[B_IS_NICHE:.+]] = icmp ult i8 %[[B_REL_DISCR]], 3
+    // 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
@@ -138,13 +138,13 @@ pub fn mid_ac_eq_discr(a: Mid<AC>, b: Mid<AC>) -> bool {
     // CHECK-LABEL: @mid_ac_eq_discr(
 
     // CHECK: %[[A_REL_DISCR:.+]] = xor i8 %a, -128
-    // CHECK: %[[A_IS_NICHE:.+]] = icmp ult i8 %[[A_REL_DISCR]], 3
+    // 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]])
     // CHECK: %[[A_DISCR:.+]] = select i1 %[[A_IS_NICHE]], i8 %[[A_REL_DISCR]], i8 1
 
     // CHECK: %[[B_REL_DISCR:.+]] = xor i8 %b, -128
-    // CHECK: %[[B_IS_NICHE:.+]] = icmp ult i8 %[[B_REL_DISCR]], 3
+    // 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]])
     // CHECK: %[[B_DISCR:.+]] = select i1 %[[B_IS_NICHE]], i8 %[[B_REL_DISCR]], i8 1
@@ -160,17 +160,17 @@ 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_REL_DISCR_WIDE:.+]] = add nsw i128 %a, -5
-    // CHECK: %[[A_REL_DISCR:.+]] = trunc nsw i128 %[[A_REL_DISCR_WIDE]] to i64
-    // CHECK: %[[A_IS_NICHE:.+]] = icmp ult i128 %[[A_REL_DISCR_WIDE]], 3
-    // CHECK: %[[A_NOT_HOLE:.+]] = icmp ne i128 %[[A_REL_DISCR_WIDE]], 1
+    // CHECK: %[[A_TRUNC:.+]] = trunc nuw nsw i128 %a to i64
+    // CHECK: %[[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
 
-    // CHECK: %[[B_REL_DISCR_WIDE:.+]] = add nsw i128 %b, -5
-    // CHECK: %[[B_REL_DISCR:.+]] = trunc nsw i128 %[[B_REL_DISCR_WIDE]] to i64
-    // CHECK: %[[B_IS_NICHE:.+]] = icmp ult i128 %[[B_REL_DISCR_WIDE]], 3
-    // CHECK: %[[B_NOT_HOLE:.+]] = icmp ne i128 %[[B_REL_DISCR_WIDE]], 1
+    // CHECK: %[[B_TRUNC:.+]] = trunc nuw nsw i128 %b to i64
+    // CHECK: %[[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
 
@@ -181,23 +181,19 @@ pub fn mid_giant_eq_discr(a: Mid<Giant>, b: Mid<Giant>) -> bool {
 
 // In niche-encoded enums, testing for the untagged variant should optimize to a
 // straight-forward comparison looking for the natural range of the payload value.
-// FIXME: A bunch don't, though.
 
 #[unsafe(no_mangle)]
 pub fn mid_bool_is_thing(a: Mid<bool>) -> bool {
     // CHECK-LABEL: @mid_bool_is_thing(
-
-    // CHECK: %[[REL_DISCR:.+]] = add nsw i8 %a, -2
-    // CHECK: %[[R:.+]] = icmp ugt i8 %[[REL_DISCR]], 2
+    // CHECK: %[[R:.+]] = icmp samesign ult i8 %a, 2
     // CHECK: ret i1 %[[R]]
     discriminant_value(&a) == 1
 }
 
 #[unsafe(no_mangle)]
 pub fn mid_ord_is_thing(a: Mid<Ordering>) -> bool {
     // CHECK-LABEL: @mid_ord_is_thing(
-    // CHECK: %[[REL_DISCR:.+]] = add nsw i8 %a, -2
-    // CHECK: %[[R:.+]] = icmp ugt i8 %[[REL_DISCR]], 2
+    // CHECK: %[[R:.+]] = icmp slt i8 %a, 2
     // CHECK: ret i1 %[[R]]
     discriminant_value(&a) == 1
 }
@@ -221,11 +217,7 @@ pub fn mid_ac_is_thing(a: Mid<AC>) -> bool {
 #[unsafe(no_mangle)]
 pub fn mid_giant_is_thing(a: Mid<Giant>) -> bool {
     // CHECK-LABEL: @mid_giant_is_thing(
-    // CHECK: %[[REL_DISCR_WIDE:.+]] = add nsw i128 %a, -5
-    // CHECK: %[[REL_DISCR:.+]] = trunc nsw i128 %[[REL_DISCR_WIDE]] to i64
-    // CHECK: %[[NOT_NICHE:.+]] = icmp ugt i128 %[[REL_DISCR_WIDE]], 2
-    // CHECK: %[[IS_MID_VARIANT:.+]] = icmp eq i64 %[[REL_DISCR]], 1
-    // CHECK: %[[R:.+]] = select i1 %[[NOT_NICHE]], i1 true, i1 %[[IS_MID_VARIANT]]
+    // CHECK: %[[R:.+]] = icmp samesign ult i128 %a, 5
     // CHECK: ret i1 %[[R]]
     discriminant_value(&a) == 1
 }
diff --git a/tests/codegen/enum/enum-match.rs b/tests/codegen/enum/enum-match.rs
@@ -41,7 +41,7 @@ pub enum Enum1 {
 // CHECK-NEXT: start:
 // 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 ult i8 %[[REL_VAR]], 2
+// CHECK-NEXT: %[[IS_NICHE:.+]] = icmp{{( samesign)?}} ugt i8 %0, 1
 // CHECK-NEXT: %[[NICHE_DISCR:.+]] = add nuw nsw i64 %[[REL_VAR_WIDE]], 1
 // CHECK-NEXT: %[[DISCR:.+]] = select i1 %[[IS_NICHE]], i64 %[[NICHE_DISCR]], i64 0
 // CHECK-NEXT: switch i64 %[[DISCR]]
@@ -148,10 +148,10 @@ pub enum MiddleNiche {
 // 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: %[[IS_NICHE:.+]] = icmp ult i8 %[[REL_VAR]], 5
 // CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %[[REL_VAR]], 2
 // CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
-// CHECK-NEXT: %[[DISCR:.+]] = select i1 %[[IS_NICHE]], i8 %[[REL_VAR]], i8 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]]
 #[no_mangle]
 pub fn match4(e: MiddleNiche) -> u8 {
@@ -167,11 +167,10 @@ pub fn match4(e: MiddleNiche) -> u8 {
 
 // CHECK-LABEL: define{{.+}}i1 @match4_is_c(i8{{.+}}%e)
 // CHECK-NEXT: start
-// CHECK-NEXT: %[[REL_VAR:.+]] = add{{( nsw)?}} i8 %e, -2
-// CHECK-NEXT: %[[NOT_NICHE:.+]] = icmp ugt i8 %[[REL_VAR]], 4
-// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %[[REL_VAR]], 2
+// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %e, 4
 // CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
-// CHECK-NEXT: ret i1 %[[NOT_NICHE]]
+// CHECK-NEXT: %[[IS_C:.+]] = icmp{{( samesign)?}} ult i8 %e, 2
+// CHECK-NEXT: ret i1 %[[IS_C]]
 #[no_mangle]
 pub fn match4_is_c(e: MiddleNiche) -> bool {
     // Before #139098, this couldn't optimize out the `select` because it looked
@@ -453,10 +452,10 @@ pub enum HugeVariantIndex {
 // CHECK-NEXT: start:
 // 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 ult i8 %[[REL_VAR]], 3
-// CHECK-NEXT: %[[NOT_IMPOSSIBLE:.+]] = icmp ne i8 %[[REL_VAR]], 1
-// CHECK-NEXT: call void @llvm.assume(i1 %[[NOT_IMPOSSIBLE]])
+// 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,
