From 6150030e8b8ecd81f56aa6b8f06af663caf71fd5 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <campbell@mumble.net>
Date: Tue, 15 Jan 2019 03:17:35 +0000
Subject: [PATCH] Make RTL:CONSTANT-COST always return positive.
Otherwise CSE might substitute constants for registers where at best
it's not helpful and at worst we don't have rules for it.
---
src/compiler/machines/aarch64/machine.scm | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/src/compiler/machines/aarch64/machine.scm b/src/compiler/machines/aarch64/machine.scm
index 01562ecf4..6548e90d4 100644
--- a/src/compiler/machines/aarch64/machine.scm
+++ b/src/compiler/machines/aarch64/machine.scm
@@ -397,18 +397,22 @@ USA.
(error "Unknown interpreter register:" locative)))
(define (rtl:constant-cost expression)
+ ;; Register reference costs 1. If a cost is less than 1, CSE will
+ ;; always choose the constant over the register reference, which may
+ ;; not always work and is not always helpful even when it does work.
+ ;;
;; XXX Justify this by reference to cycle counts, &c. This really
;; depends on which instruction we're talking about -- sometimes
;; immediates are cheaper.
- (let ((cost:zero 0)
- (cost:imm16 1) ;MOVZ/MOVN
- (cost:imm32 2) ;MOVZ/MOVN + 1*MOVK
- (cost:imm48 3) ;MOVZ/MOVN + 2*MOVK
- (cost:imm64 4) ;MOVZ/MOVN + 3*MOVK
- (cost:add 1)
- (cost:adr 1)
+ (let ((cost:zero 1)
+ (cost:imm16 2) ;MOVZ/MOVN
+ (cost:imm32 3) ;MOVZ/MOVN + 1*MOVK
+ (cost:imm48 4) ;MOVZ/MOVN + 2*MOVK
+ (cost:imm64 5) ;MOVZ/MOVN + 3*MOVK
+ (cost:add 2)
+ (cost:adr 2)
(cost:ldr 10)
- (cost:bl 2))
+ (cost:bl 3))
(define (immediate-cost immediate)
(cond ((zero? immediate)
cost:zero)
--
2.25.1