The build only stays a working game when code changes size if every address in the code and data is a symbol, which the linker moves with what it names. Three checks guard it; CONTRIBUTING.md says how to fix what they find.

shiftcheck

The links keep their relocations (--emit-relocs), and make shiftcheck (tools/shiftcheck.py) reads them to find the addresses that are numbers: a raw 0x8... in C, a .word that splat didn't symbolize, a pointer in a u32 table, a lui or a jal without a relocation, a symbol that a symbol file or splat's undefined_syms_auto gives an absolute address:

# Fail on any address that config/<version>/shiftcheck.txt doesn't list
make VERSION=eu shiftcheck
 
# List every one, with what it points to and where it is
make VERSION=eu shiftreport

A binary that points into the binaries it loads links against their symbols: the executable against the mode overlays (MODE_ENTRY_POINTS), FIGHTSTG against WFIGHTMN and WFIGHTTS, FIELDSTG against the stages, whose names it takes prefixed with the stage's (WSTAG931_startStage, include/field/stages.h). CHILDREN_<binary> in the Makefile lists them; tools/link_imports.py writes build/<version>/<binary>_imports.ld with the names the binary uses and doesn't define, and the children link against the binary's own symbols from a first link of it with placeholders for those names (build/<version>/layout/). config/<version>/shiftcheck.txt lists the addresses still to fix, and is empty in both versions. FIELDSTG's event and script command tables, which point into whichever stage is loaded, name the symbols of the stage each entry belongs to: WSTAG260_script1320, one of the story events' scripts, and WSTAG780_createCommand800, a function of the stage whose scripts run the command. The list holds an address by where it is and what it holds, with the names after a #, so that a rename leaves it as it is. make shiftcheck warns about those that are fixed (and tools/shiftcheck.py --strict fails), which tools/shiftcheck.py -v <version> --update drops. The memory map (EXE_VRAM, OVERLAY_VRAM and STAGE_VRAM in mk/version/<version>.mk, where the executable and the overlays load) is fixed by design, and the binaries link at its symbols, not at numbers.

padcheck

A padding build links every binary again from the same objects with the memory map moved up, as if the executable had grown at its start, into build/<version>/pad<pad>/; in it, a word may only change where it has a relocation (tools/padcheck.py):

# Padding builds at +0x4 and +0x10004 (which carries into the upper half of
# every %hi/%lo pair), each checked against the build
make VERSION=eu padcheck

make PAD=0x10004 smoke boots a padding build in DuckStation, on your machine only (CONTRIBUTING.md).

inputcheck

Each link also checks what it took in (tools/inputcheck.py): only the objects the Makefile built for the binary, as a linker script would link a stale object that is still in build/, and no blob, whose words have no relocations, but the executable's tail, a picture.

In the CI

The CI runs make padcheck, and tools/shiftcheck.py --strict --base HEAD^, which also fails when the list has an entry that is fixed, or one that the parent commit's doesn't have: the list may only get shorter.