From c0166f78f9f9954320b52ef3be258b97410ef15c Mon Sep 17 00:00:00 2001
From: Karl Meakin <karlwfmeakin@gmail.com>
Date: Tue, 11 Apr 2023 23:41:19 +0100
Subject: [PATCH] ISLE: simplify select/bitselect when both choices are the
 same (#6141)

---
 cranelift/codegen/src/opts/selects.isle          |  4 ++++
 cranelift/filetests/filetests/egraph/select.clif | 14 ++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/cranelift/codegen/src/opts/selects.isle b/cranelift/codegen/src/opts/selects.isle
index c8fbc09a90..1fcc6c8d64 100644
--- a/cranelift/codegen/src/opts/selects.isle
+++ b/cranelift/codegen/src/opts/selects.isle
@@ -1,5 +1,9 @@
 ;; `select`/`bitselect`-related rewrites
 
+;; remove select when both choices are the same
+(rule (simplify (select    ty _ x x)) x)
+(rule (simplify (bitselect ty _ x x)) x)
+
 ;; Transform select-of-icmp into {u,s}{min,max} instructions where possible.
 (rule (simplify (select ty (sgt _ x y) x y)) (smax ty x y))
 (rule (simplify (select ty (sge _ x y) x y)) (smax ty x y))
diff --git a/cranelift/filetests/filetests/egraph/select.clif b/cranelift/filetests/filetests/egraph/select.clif
index e8a225b4de..0e7a2be288 100644
--- a/cranelift/filetests/filetests/egraph/select.clif
+++ b/cranelift/filetests/filetests/egraph/select.clif
@@ -5,6 +5,20 @@ target aarch64
 target s390x
 target riscv64
 
+function %select_self(i8, i32) -> i32 {
+block0(v0: i8, v1: i32):
+    v2 = select v0, v1, v1
+    return v2
+    ; check: return v1
+}
+
+function %bitselect_self(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+    v2 = bitselect v0, v1, v1
+    return v2
+    ; check: return v1
+}
+
 function %select_sgt_to_smax(i32, i32) -> i32 {
 block0(v0: i32, v1: i32):
     v2 = icmp sgt v0, v1