From 7573e40d57e15abca8a29f9907d29f2d4a97c119 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 4 Mar 2026 18:12:54 +1300 Subject: [PATCH v2 01/19] Provide PG_STACK_DIRECTION configuration macro. General purpose systems capable of running PostgreSQL with stacks that grow upwards rather down downwards don't seem to be found in the wild these days. Define a pg_config_manual.h variable that can be set to -1 or 1, to remove runtime branching from stack-measuring code. We could always add feature detection if a new system shows up that wants the other setting, but runtime doesn't make sense for a fixed properly of a system. Though the existing user of that fact is very simple and doesn't save much effort at all, a proposed patch would also like to know this at compile time. --- src/backend/utils/misc/stack_depth.c | 10 ++++++++-- src/include/pg_config_manual.h | 8 ++++++++ 2 files changed, 16 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/misc/stack_depth.c b/src/backend/utils/misc/stack_depth.c index 61a07cf824e..3cd1b6f2336 100644 --- a/src/backend/utils/misc/stack_depth.c +++ b/src/backend/utils/misc/stack_depth.c @@ -118,9 +118,15 @@ stack_is_too_deep(void) /* * Take abs value, since stacks grow up on some machines, down on others + * (historical). This formulation amounts to a no-op on modern systems. */ - if (stack_depth < 0) - stack_depth = -stack_depth; + stack_depth *= -(PG_STACK_DIRECTION); + + /* + * If this assertion fails, either PG_STACK_DIRECTION is wrong or this + * system doesn't have a traditional stack. + */ + Assert(stack_depth >= 0); /* * Trouble? diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h index 521b49b8888..75035b9e796 100644 --- a/src/include/pg_config_manual.h +++ b/src/include/pg_config_manual.h @@ -222,6 +222,14 @@ */ #define PG_IO_ALIGN_SIZE 4096 +/* + * Historical systems had stacks that grew upwards in address space, but none + * of them remain. Our stack depth-checking code no longer wastes cycles + * considering that possibility at runtime. It assumes (but also asserts) + * that the stack grows in the direction defined here: -1 or +1. + */ +#define PG_STACK_DIRECTION -1 + /* *------------------------------------------------------------------------ * The following symbols are for enabling debugging code, not for -- 2.53.0