aco: rename README to README.md

Closes: #1974
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>

1 files changed, 0 insertions, 211 deletions

diff --git a/src/amd/compiler/README b/src/amd/compiler/README
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-# Unofficial GCN/RDNA ISA reference errata
-
-## v_sad_u32
-
-The Vega ISA reference writes it's behaviour as:
-```
-D.u = abs(S0.i - S1.i) + S2.u.
-```
-This is incorrect. The actual behaviour is what is written in the GCN3 reference
-guide:
-```
-ABS_DIFF (A,B) = (A>B) ? (A-B) : (B-A)
-D.u = ABS_DIFF (S0.u,S1.u) + S2.u
-```
-The instruction doesn't subtract the S0 and S1 and use the absolute value (the
-_signed_ distance), it uses the _unsigned_ distance between the operands. So
-`v_sad_u32(-5, 0, 0)` would return `4294967291` (`-5` interpreted as unsigned),
-not `5`.
-
-## s_bfe_*
-
-Both the Vega and GCN3 ISA references write that these instructions don't write
-SCC. They do.
-
-## v_bcnt_u32_b32
-
-The Vega ISA reference writes it's behaviour as:
-```
-D.u = 0;
-for i in 0 ... 31 do
-D.u += (S0.u[i] == 1 ? 1 : 0);
-endfor.
-```
-This is incorrect. The actual behaviour (and number of operands) is what
-is written in the GCN3 reference guide:
-```
-D.u = CountOneBits(S0.u) + S1.u.
-```
-
-## SMEM stores
-
-The Vega ISA references doesn't say this (or doesn't make it clear), but
-the offset for SMEM stores must be in m0 if IMM == 0.
-
-The RDNA ISA doesn't mention SMEM stores at all, but they seem to be supported
-by the chip and are present in LLVM. AMD devs however highly recommend avoiding
-these instructions.
-
-## SMEM atomics
-
-RDNA ISA: same as the SMEM stores, the ISA pretends they don't exist, but they
-are there in LLVM.
-
-## VMEM stores
-
-All reference guides say (under "Vector Memory Instruction Data Dependencies"):
-> When a VM instruction is issued, the address is immediately read out of VGPRs
-> and sent to the texture cache. Any texture or buffer resources and samplers
-> are also sent immediately. However, write-data is not immediately sent to the
-> texture cache.
-Reading that, one might think that waitcnts need to be added when writing to
-the registers used for a VMEM store's data. Experimentation has shown that this
-does not seem to be the case on GFX8 and GFX9 (GFX6 and GFX7 are untested). It
-also seems unlikely, since NOPs are apparently needed in a subset of these
-situations.
-
-## MIMG opcodes on GFX8/GCN3
-
-The `image_atomic_{swap,cmpswap,add,sub}` opcodes in the GCN3 ISA reference
-guide are incorrect. The Vega ISA reference guide has the correct ones.
-
-## VINTRP encoding
-
-VEGA ISA doc says the encoding should be `110010` but `110101` works.
-
-## VOP1 instructions encoded as VOP3
-
-RDNA ISA doc says that `0x140` should be added to the opcode, but that doesn't
-work. What works is adding `0x180`, which LLVM also does.
-
-## FLAT, Scratch, Global instructions
-
-The NV bit was removed in RDNA, but some parts of the doc still mention it.
-
-RDNA ISA doc 13.8.1 says that SADDR should be set to 0x7f when ADDR is used, but
-9.3.1 says it should be set to NULL. We assume 9.3.1 is correct and set it to
-SGPR_NULL.
-
-## Legacy instructions
-
-Some instructions have a `_LEGACY` variant which implements "DX9 rules", in which
-the zero "wins" in multiplications, ie. `0.0*x` is always `0.0`. The VEGA ISA
-mentions `V_MAC_LEGACY_F32` but this instruction is not really there on VEGA.
-
-## RDNA L0, L1 cache and DLC, GLC bits
-
-The old L1 cache was renamed to L0, and a new L1 cache was added to RDNA. The
-L1 cache is 1 cache per shader array. Some instruction encodings have DLC and
-GLC bits that interact with the cache.
-
-* DLC ("device level coherent") bit: controls the L1 cache
-* GLC ("globally coherent") bit: controls the L0 cache
-
-The recommendation from AMD devs is to always set these two bits at the same time,
-as it doesn't make too much sense to set them independently, aside from some
-circumstances (eg. we needn't set DLC when only one shader array is used).
-
-Stores and atomics always bypass the L1 cache, so they don't support the DLC bit,
-and it shouldn't be set in these cases. Setting the DLC for these cases can result
-in graphical glitches.
-
-## RDNA S_DCACHE_WB
-
-The S_DCACHE_WB is not mentioned in the RDNA ISA doc, but it is needed in order
-to achieve correct behavior in some SSBO CTS tests.
-
-## RDNA subvector mode
-
-The documentation of S_SUBVECTOR_LOOP_BEGIN and S_SUBVECTOR_LOOP_END is not clear
-on what sort of addressing should be used, but it says that it
-"is equivalent to an S_CBRANCH with extra math", so the subvector loop handling
-in ACO is done according to the S_CBRANCH doc.
-
-# Hardware Bugs
-
-## SMEM corrupts VCCZ on SI/CI
-
-https://github.com/llvm/llvm-project/blob/acb089e12ae48b82c0b05c42326196a030df9b82/llvm/lib/Target/AMDGPU/SIInsertWaits.cpp#L580-L616
-After issuing a SMEM instructions, we need to wait for the SMEM instructions to
-finish and then write to vcc (for example, `s_mov_b64 vcc, vcc`) to correct vccz
-
-Currently, we don't do this.
-
-## RDNA / GFX10 hazards
-
-### SMEM store followed by a load with the same address
-
-We found that an `s_buffer_load` will produce incorrect results if it is preceded
-by an `s_buffer_store` with the same address. Inserting an `s_nop` between them
-does not mitigate the issue, so an `s_waitcnt lgkmcnt(0)` must be inserted.
-This is not mentioned by LLVM among the other GFX10 bugs, but LLVM doesn't use
-SMEM stores, so it's not surprising that they didn't notice it.
-
-### VMEMtoScalarWriteHazard
-
-Triggered by:
-VMEM/FLAT/GLOBAL/SCRATCH/DS instruction reads an SGPR (or EXEC, or M0).
-Then, a SALU/SMEM instruction writes the same SGPR.
-
-Mitigated by:
-A VALU instruction or an `s_waitcnt vmcnt(0)` between the two instructions.
-
-### SMEMtoVectorWriteHazard
-
-Triggered by:
-An SMEM instruction reads an SGPR. Then, a VALU instruction writes that same SGPR.
-Despite LLVM
-
-Mitigated by:
-Any non-SOPP SALU instruction (except `s_setvskip`, `s_version`, and any non-lgkmcnt `s_waitcnt`).
-
-### Offset3fBug
-
-Any branch that is located at offset 0x3f will be buggy. Just insert some NOPs to make sure no branch
-is located at this offset.
-
-### InstFwdPrefetchBug
-
-According to LLVM, the `s_inst_prefetch` instruction can cause a hang.
-There are no further details.
-
-### LdsMisalignedBug
-
-When there is a misaligned multi-dword FLAT load/store instruction in WGP mode,
-it needs to be split into multiple single-dword FLAT instructions.
-
-ACO doesn't use FLAT load/store on GFX10, so is unaffected.
-
-### FlatSegmentOffsetBug
-
-The 12-bit immediate OFFSET field of FLAT instructions must always be 0.
-GLOBAL and SCRATCH are unaffected.
-
-ACO doesn't use FLAT load/store on GFX10, so is unaffected.
-
-### VcmpxPermlaneHazard
-
-Triggered by:
-Any permlane instruction that follows any VOPC instruction.
-Confirmed by AMD devs that despite the name, this doesn't only affect v_cmpx.
-
-Mitigated by: any VALU instruction except `v_nop`.
-
-### VcmpxExecWARHazard
-
-Triggered by:
-Any non-VALU instruction reads the EXEC mask. Then, any VALU instruction writes the EXEC mask.
-
-Mitigated by:
-A VALU instruction that writes an SGPR (or has a valid SDST operand), or `s_waitcnt_depctr 0xfffe`.
-Note: `s_waitcnt_depctr` is an internal instruction, so there is no further information
-about what it does or what its operand means.
-
-### LdsBranchVmemWARHazard
-
-Triggered by:
-VMEM/GLOBAL/SCRATCH instruction, then a branch, then a DS instruction,
-or vice versa: DS instruction, then a branch, then a VMEM/GLOBAL/SCRATCH instruction.
-
-Mitigated by:
-Only `s_waitcnt_vscnt null, 0`. Needed even if the first instruction is a load.