docs(technical-debt): state the measured LDS atomic overlap as the slot's high dword

docs/dev/TechnicalDebt.md

@@ -454,7 +454,7 @@ Throughout the project, every function at every stage either **does exactly what
 - [Converted unorm images](../../core/shader/recompiler/SpirvBackend/src/SpirvImageEmitter.cpp) (10_11_11_UNORM, read and written through an R32_UINT view): samples, gathers, LOD queries, 16-bit data and a DST_SEL of the missing W channel throw.
 - [Packed 16-bit textures](../../core/libs/prx/libSceAgcDriver/Graphics/src/TextureFormat.cpp) (libSceAgcDriver) - `5_6_5`, `1_5_5_5` and `4_4_4_4` are viewed as R5G6B5, A1R5G5B5 and R4G4B4A4 with a swizzle that reads X from the low bits, and a DST_SEL of the missing W of `5_6_5` reads X. Measured on RDNA2 (gfx1035), not on the console: through `image_load` and `image_sample_lz`, X, Y, Z (and W of `1_5_5_5` and `4_4_4_4`) come from the low, middle and high fields, and W of `5_6_5` returns X (the low 5 bits), not 1.
 - [Converted unorm image loads](../../core/shader/recompiler/SpirvBackend/src/SpirvImageEmitter.cpp) divide by 2^n - 1 with the buffer format path's `OpFDiv`, which Vulkan allows to be up to 2.5 ULP off, so some drivers are not bit-exact.
-- [64-bit LDS atomics](../../core/shader/recompiler/SpirvBackend/src/SpirvMemory/SpirvMemoryInstructions.cpp) (`ds_*_u64`, `ds_*_b64`, `ds_*_f64`) read and write the two dwords under a workgroup lock stored after the guest LDS, as LDS is a 32-bit array without 64-bit atomics. On RDNA2 (gfx1036) the LDS serialises a 32-bit and a 64-bit atomic on the same address: 128 lanes of `ds_add_u32` and 128 lanes of `ds_add_u64` on one slot sum exactly. Only that pair was measured. Every 32-bit LDS atomic of a program that contains a 64-bit LDS atomic therefore takes the same lock (the other 32-bit LDS atomics, `ds_min/max`, `ds_cmpst`, the float forms, `ds_inc/dec`, `ds_mskor` and `ds_wrxchg`, are extrapolated from `ds_add_u32` and were not measured against a 64-bit atomic); a program without a 64-bit LDS atomic keeps its native 32-bit atomics. `ds_condxchg32_rtn_b64` is two `ds_mskor` halves, each under the lock but not atomic as a pair, so a 64-bit atomic between the two halves is not excluded. Sub-dword writes (`ds_write_b8`, `ds_write_b16` and their `_d16_hi` forms) and plain `ds_read_*`, `ds_write_*`, `ds_append_u32` and `ds_consume_u32` are not taken under the lock, so a plain access racing a 64-bit atomic on the same dwords can see or overwrite one half of it; whether the hardware orders them was not measured. The lock serialises the workgroup's LDS atomics of such a program; the throughput cost was not measured. `VK_KHR_workgroup_memory_explicit_layout` with `shaderSharedInt64Atomics` would make both widths native and is not used. A 64-bit atomic at an address that is 4-byte but not 8-byte aligned faults the wave on gfx1036, as on gfx1035.
+- [64-bit LDS atomics](../../core/shader/recompiler/SpirvBackend/src/SpirvMemory/SpirvMemoryInstructions.cpp) (`ds_*_u64`, `ds_*_b64`, `ds_*_f64`) read and write the two dwords under a workgroup lock stored after the guest LDS, as LDS is a 32-bit array without 64-bit atomics. On RDNA2 (gfx1036) the LDS serialises a 32-bit atomic on a slot's high dword and a 64-bit atomic on that slot: 128 lanes of `ds_add_u32` and 128 lanes of `ds_add_u64` on one slot sum exactly. Only that pair was measured. Every 32-bit LDS atomic of a program that contains a 64-bit LDS atomic therefore takes the same lock (the other 32-bit LDS atomics, `ds_min/max`, `ds_cmpst`, the float forms, `ds_inc/dec`, `ds_mskor` and `ds_wrxchg`, are extrapolated from `ds_add_u32` and were not measured against a 64-bit atomic); a program without a 64-bit LDS atomic keeps its native 32-bit atomics. `ds_condxchg32_rtn_b64` is two `ds_mskor` halves, each under the lock but not atomic as a pair, so a 64-bit atomic between the two halves is not excluded. Sub-dword writes (`ds_write_b8`, `ds_write_b16` and their `_d16_hi` forms) and plain `ds_read_*`, `ds_write_*`, `ds_append_u32` and `ds_consume_u32` are not taken under the lock, so a plain access racing a 64-bit atomic on the same dwords can see or overwrite one half of it; whether the hardware orders them was not measured. The lock serialises the workgroup's LDS atomics of such a program; the throughput cost was not measured. `VK_KHR_workgroup_memory_explicit_layout` with `shaderSharedInt64Atomics` would make both widths native and is not used. A 64-bit atomic at an address that is 4-byte but not 8-byte aligned faults the wave on gfx1036, as on gfx1035.
 - [LDS float atomics](../../core/shader/recompiler/SpirvBackend/src/SpirvMemory/SpirvMemoryInstructions.cpp) (`ds_min/max/cmpst_f32`, `ds_min/max/cmpst_f64` and their `_rtn` forms) follow the hardware measured on RDNA2 (gfx1036) over 14 x 14 operand sets (zeros, ones, infinities, denormals, quiet and signalling NaNs of both signs and with payloads) in IEEE 0 and 1 with every value 0-3 of the type's denormal field: both operands are flushed to a signed zero when bit 0 of the field is clear (0 and 2; 1 and 3 keep them; the f32 field for the f32 forms, the f16/f64 field for the f64 forms; a shader without a known float mode flushes f32 and keeps f64), then a signalling-NaN data operand is stored quieted, else a signalling-NaN old value is stored quieted, else a quiet-NaN old value is replaced by the data, else a quiet-NaN data operand loses, else the ordered compare with -0 below +0 decides, and the flushed value is what is written, also when the old value wins. `cmpst` compares the flushed bits (both zeros equal, NaN never equal) and leaves memory unchanged when they differ. The returned value is always the old value as it was in memory. `IEEE_MODE` has no effect. The same bit rule serves the buffer float atomics (`AtomicFloatBits`). Not measured: a denormal or signalling-NaN `data1` of `cmpst` (stored as given), and `ds_add_f32`, which keeps the host's `OpFAdd`.
 - [V# bases off a DWORD boundary](../../core/shader/recompiler/SpirvBackend/src/SpirvMemory/SpirvMemoryInstructions.cpp) (shader recompiler) - raw accesses use the exact bytes and `s_buffer_load` reads `(base & ~3) + (offset & ~3)`, as measured on gfx1035 (Linux, alignment mode UNALIGNED; the console's mode is unknown). Formatted accesses off their element's alignment and atomics throw.
 - [32-bit bitfield masks](../../core/shader/recompiler/Translation/src/IntegerInstructions.cpp) (`S_BFM_B32`, `V_BFM_B32`) use the truncating 32-bit shift specified by the [RDNA 1.0 ISA](https://www.amd.com/content/dam/amd/en/documents/radeon-tech-docs/instruction-set-architectures/rdna-shader-instruction-set-architecture.pdf), sections 12.1 (opcode 36) and 12.12 (opcode 867). Width and offset use their low five bits. This matches gfx1035 (Linux) for every width/offset pair, with and without upper operand bits set, and both leave SCC unchanged; not measured on the console.