From ed338d000a874241032b3d553440d1ac75c3dd21 Mon Sep 17 00:00:00 2001
From: Keno Fischer <keno@juliacomputing.com>
Date: Mon, 26 Jun 2023 03:27:33 -0400
Subject: [PATCH] lattice: Restore `tmerge_fast_path` for optimizer lattice
 (#50291)

This function is slightly misnamed, because, while it does perform
some fast pathing, the rest of the tmerge functions assume it has
been run. When I removed the OptimizerLattice in #50257, we
accidentally lost the call to `tmerge_fast_path` in the optimizer_lattice
path, regressing tmerge quality in the optimizer (which admittedly
we don't use a lot, but I did just add another use for in #50287).
The proper fix is probably to split the tmerge into a helper that
always calls the fast path and then delegates to the current
implementation for everything else, but for now, just try moving
the fast path to the start of the PartialsLattice, which should
restore correctness and have reasonable performance, since
there's only a handful of pointer checks between the entry to the
inference tmerge and the partials lattice.
---
 base/compiler/typelimits.jl | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/base/compiler/typelimits.jl b/base/compiler/typelimits.jl
index 81a176dff1c9d..b648144ea3bd1 100644
--- a/base/compiler/typelimits.jl
+++ b/base/compiler/typelimits.jl
@@ -456,8 +456,6 @@ end
         return tmerge_limited(lattice, typea, typeb)
     end
 
-    r = tmerge_fast_path(widenlattice(lattice), typea, typeb)
-    r !== nothing && return r
     return tmerge(widenlattice(lattice), typea, typeb)
 end
 
@@ -491,8 +489,13 @@ end
         end
         return Bool
     end
-    typea = widenconditional(typea)
-    typeb = widenconditional(typeb)
+    if isa(typea, Conditional)
+        typeb === Union{} && return typea
+        typea = widenconditional(typea)
+    elseif isa(typeb, Conditional)
+        typea === Union{} && return typeb
+        typeb = widenconditional(typeb)
+    end
     return tmerge(widenlattice(lattice), typea, typeb)
 end
 
@@ -526,14 +529,25 @@ end
         end
         return Bool
     end
-    typea = widenconditional(typea)
-    typeb = widenconditional(typeb)
+    if isa(typea, InterConditional)
+        typeb === Union{} && return typea
+        typea = widenconditional(typea)
+    elseif isa(typeb, InterConditional)
+        typea === Union{} && return typeb
+        typeb = widenconditional(typeb)
+    end
     return tmerge(widenlattice(lattice), typea, typeb)
 end
 
 @nospecializeinfer function tmerge(𝕃::AnyMustAliasesLattice, @nospecialize(typea), @nospecialize(typeb))
-    typea = widenmustalias(typea)
-    typeb = widenmustalias(typeb)
+    if is_valid_lattice_norec(𝕃, typea)
+        typeb === Union{} && return typea
+        typea = widenmustalias(typea)
+    end
+    if is_valid_lattice_norec(𝕃, typeb)
+        typea === Union{} && return typeb
+        typeb = widenmustalias(typeb)
+    end
     return tmerge(widenlattice(𝕃), typea, typeb)
 end
 
@@ -598,6 +612,9 @@ end
 end
 
 @nospecializeinfer function tmerge(lattice::PartialsLattice, @nospecialize(typea), @nospecialize(typeb))
+    r = tmerge_fast_path(lattice, typea, typeb)
+    r !== nothing && return r
+
     # type-lattice for Const and PartialStruct wrappers
     aps = isa(typea, PartialStruct)
     bps = isa(typeb, PartialStruct)