Rollup merge of #139098 - scottmcm:assert-impossible-tags, r=WaffleLapkin

Tell LLVM about impossible niche tags

I was trying to find a better way of emitting discriminant calculations, but sadly had no luck.

So here's a fairly small PR with the bits that did seem worth bothering:

1. As the [`TagEncoding::Niche` docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.TagEncoding.html#variant.Niche) describe, it's possible to end up with a dead value in the input that's not already communicated via the range parameter attribute nor the range load metadata attribute.  So this adds an `llvm.assume` in non-debug mode to tell LLVM about that.  (That way it can tell that the sides of the `select` have disjoint possible values.)

2. I'd written a bunch more tests, or at least made them parameterized, in the process of trying things out, so this checks in those tests to hopefully help future people not trip on the same weird edge cases, like when the tag type is `i8` but yet there's still a variant index and discriminant of `258` which doesn't fit in that tag type because the enum is really weird.

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -9,6 +9,7 @@ use rustc_middle::mir::{self, ConstValue};
 use rustc_middle::ty::Ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::{bug, span_bug};
+use rustc_session::config::OptLevel;
 use tracing::{debug, instrument};
 
 use super::place::{PlaceRef, PlaceValue};
@@ -496,6 +497,18 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                     _ => (tag_imm, bx.cx().immediate_backend_type(tag_op.layout)),
                 };
 
+                // Layout ensures that we only get here for cases where the discriminant
+                // value and the variant index match, since that's all `Niche` can encode.
+                // But for emphasis and debugging, let's double-check one anyway.
+                debug_assert_eq!(
+                    self.layout
+                        .ty
+                        .discriminant_for_variant(bx.tcx(), untagged_variant)
+                        .unwrap()
+                        .val,
+                    u128::from(untagged_variant.as_u32()),
+                );
+
                 let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
 
                 // We have a subrange `niche_start..=niche_end` inside `range`.
@@ -537,6 +550,21 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                         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);
+                    }
+
                     (is_niche, cast_tag, niche_variants.start().as_u32() as u128)
                 };
 
@@ -553,7 +581,9 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 );
 
                 // In principle we could insert assumes on the possible range of `discr`, but
-                // currently in LLVM this seems to be a pessimization.
+                // currently in LLVM this isn't worth it because the original `tag` will
+                // have either a `range` parameter attribute or `!range` metadata,
+                // or come from a `transmute` that already `assume`d it.
 
                 discr
             }