From: Taylor R Campbell <campbell@mumble.net>
Date: Fri, 25 Jan 2019 04:21:09 +0000 (+0000)
Subject: Add note on ughbletchness of ADRP.
X-Git-Tag: mit-scheme-pucked-10.1.20~11^2~66^2~19
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=85202ca21ec7fc8bc184fdc09305b83af1aba0e9;p=mit-scheme.git

Add note on ughbletchness of ADRP.
---

diff --git a/src/compiler/machines/aarch64/instr1.scm b/src/compiler/machines/aarch64/instr1.scm
index 7a2cda8f2..f55a1ad5e 100644
--- a/src/compiler/machines/aarch64/instr1.scm
+++ b/src/compiler/machines/aarch64/instr1.scm
@@ -1297,6 +1297,14 @@ USA.
 ;; Macro used for assembling variable-width instructions which can't
 ;; just defer to the variable-width ADR because we prohibit variable
 ;; width within variable width.
+;;
+;; XXX OOPS!  This doesn't actually work, because ADRP actually
+;; computes PC - (PC mod 2^12) + 2^12*offset, not PC + 2^12*offset.
+;; Either we need to know where we are in a page (which can change from
+;; GC to GC because compiled blocks are only float-aligned, not
+;; page-aligned), or we need to generate extra code to find and add PC
+;; mod 2^12, which looks like it'll require another temporary register.
+;; Ugh!
 
 (define-instruction ADRP-ADD
   ((X (? Rd) (@PCO (? offset signed-33)))