From a153feaa0b167520d6e42d1aa03d224ec0064a7c Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Fri, 4 Jan 2019 12:53:25 -0500
Subject: [PATCH] optimize identity splat of a tuple, `(t...,)` (#30571)

---
 base/compiler/ssair/inlining.jl | 6 ++++++
 src/builtins.c                  | 3 +++
 test/compiler/inline.jl         | 3 +++
 3 files changed, 12 insertions(+)

diff --git a/base/compiler/ssair/inlining.jl b/base/compiler/ssair/inlining.jl
index 418afe10fcb46..1b7cbebac5caa 100644
--- a/base/compiler/ssair/inlining.jl
+++ b/base/compiler/ssair/inlining.jl
@@ -837,6 +837,12 @@ function assemble_inline_todo!(ir::IRCode, linetable::Vector{LineInfoNode}, sv::
             # Independent of whether we can inline, the above analysis allows us to rewrite
             # this apply call to a regular call
             ft = atypes[2]
+            if length(atypes) == 3 && ft isa Const && ft.val === Core.tuple && atypes[3] ⊑ Tuple
+                # rewrite `((t::Tuple)...,)` to `t`
+                ir.stmts[idx] = stmt.args[3]
+                ok = false
+                break
+            end
             stmt.args, atypes = rewrite_apply_exprargs!(ir, idx, stmt.args, atypes, sv)
             ok = !has_free_typevars(ft)
             ok || break
diff --git a/src/builtins.c b/src/builtins.c
index 4f5938a17e574..1f67eb0e7202d 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -462,6 +462,9 @@ JL_CALLABLE(jl_f__apply)
                 return (jl_value_t*)t;
             }
         }
+        else if (f == jl_builtin_tuple && jl_is_tuple(args[1])) {
+            return args[1];
+        }
     }
     size_t n=0, i, j;
     for(i=1; i < nargs; i++) {
diff --git a/test/compiler/inline.jl b/test/compiler/inline.jl
index 47d770977f016..c7e55ac19fe03 100644
--- a/test/compiler/inline.jl
+++ b/test/compiler/inline.jl
@@ -217,3 +217,6 @@ function f_div(x, y)
     return x
 end
 @test length(code_typed(f_div, (Int, Int))[1][1].code) > 1
+
+f_identity_splat(t) = (t...,)
+@test length(code_typed(f_identity_splat, (Tuple{Int,Int},))[1][1].code) == 1