From: Kirill A. Korinsky <kirill@korins.ky>
Subject: sys/alpha: allign _STACKALIGNBYTES with GCC expectation
To: OpenBSD tech <tech@openbsd.org>
Date: Wed, 14 Oct 2026 17:28:21 +0200

tech@,

martijn@ had tried to build gcc-16 on his alpha and shared with me that it
crashed as:

build/genpreds -h /usr/ports/pobj/gcc-11.2.0/gcc-11.2.0/gcc/common.md /usr/ports/pobj/gcc-11.2.0/gcc-11.2.0/gcc/config/alpha/alpha.md > tmp-preds.h
Segmentation fault (core dumped)

he had shared with me the pobj's folder with .core where I saw:

(gdb) bt
Python Exception <class 'gdb.MemoryError'>: Cannot access memory at address 0x14
Cannot access memory at address 0x14
(gdb)

which smells like broken or truncated stack with almost NULL.

He confirmed that gcc-8 builds and gcc-16 doesn't which is far larger.

I recall that gcc has it's own stack size as part of system definition.

Here I just made an assumption that kernel truncate stack before gcc thinks
it will and run grep -r STACK by sys/arch/alpha it bring me hit with
_STACKALIGNBYTES which I had compared agains't gcc's alpha definition and
saw missmatch. Checked gcc in base, smae missmatch. Checked NetBSD the same
missmatch.

So, NetBSD, modern GCC and GCC in base defines it the same way with 128 bit mask:

https://github.com/NetBSD/src/blob/trunk/sys/arch/alpha/include/param.h#L60-L64

https://github.com/gcc-mirror/gcc/blob/master/gcc/config/alpha/alpha.h#L253-L254

https://github.com/openbsd/src/blob/master/gnu/gcc/gcc/config/alpha/alpha.h#L352-L353

And we defines it with 64 bit mask:
https://github.com/openbsd/src/blob/master/sys/arch/alpha/include/_types.h#L54-L55

Here a diff whcih allign _STACKALIGNBYTES definition in base with what gcc
expects.

martijn@ had tested it and confirmed that it helped.

Ok?


diff --git sys/arch/alpha/include/_types.h sys/arch/alpha/include/_types.h
index 99f091ebe6b..56fefc5f6da 100644
--- sys/arch/alpha/include/_types.h
+++ sys/arch/alpha/include/_types.h
@@ -52,7 +52,7 @@ typedef struct label_t {
  * (within reasonable limits).
  */
 #define	_ALIGNBYTES		7
-#define	_STACKALIGNBYTES	_ALIGNBYTES
+#define	_STACKALIGNBYTES	15
 #define	_ALIGN(p)		(((unsigned long)(p) + _ALIGNBYTES) & ~_ALIGNBYTES)
 #define	_ALIGNED_POINTER(p,t)	((((unsigned long)(p)) & (sizeof(t) - 1)) == 0)
 #define	_MAX_PAGE_SHIFT		13	/* same as PAGE_SHIFT */


-- 
wbr, Kirill