Reading src_shared_base, src_shared_limit, src_private_base or
src_private_limit (scalar source codes 235-238) refused the shader since
#1940, because their values were unknown. They now read as the 64-bit
apertures that the flat routing from #1190 classifies: a base is the
aperture's high dword over a low dword of 0, a limit is the same high
dword over 0xffffffff (shared 0x80000000, private 0x70000000).
A 64-bit read (s_mov_b64, a 64-bit VALU source, a wave64 lane mask)
gets the whole value and a 32-bit read gets the low dword. A flat
pointer built from the high dword therefore reaches LDS or scratch, and
64-bit compares against the limits order as on the hardware. The flat
routing and the reads share one definition of the two high dwords.
Sources: LLVM SIRegisterInfo.td (aperture registers are 64-bit, their
low 32 bits are always 0 for a base and -1 for a limit) and
getSegmentAperture in AMDGPULegalizerInfo.cpp and SIISelLowering.cpp
(a 64-bit read whose high half is the aperture); the RDNA 2 ISA guide,
sections 5.2, 5.8 (SH_MEM_BASES holds 16-bit bases) and 9.3; the KFD's
MAKE_LDS_APP_LIMIT (limit = base | 0xffffffff); the gfx1030 and gfx1035
readouts on #1190 and #3803. The PS5's two high dwords stay inferred,
as the shader technical debt records.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
