Add IBM POWER8 (ppc64le) support with VSX SIMD I2_S kernels

[Scottcjn]

Summary

Adds IBM POWER8 (ppc64le) support with scalar and VSX SIMD I2_S kernels. The VSX implementation uses vmsummbm for 16-way signed-unsigned byte multiply-accumulate and dcbt L3-resident prefetch hints for weight tensor caching.

Achieves 9.1–10.5x speedup over scalar baseline on prompt processing across three model sizes (700M, 2B, 8B).

Also adds Power Mac G5 (big-endian PowerPC 970) support with AltiVec SIMD kernels via VSX/AltiVec compatibility macros. One code path works on both POWER8 and G5 — 4 macros abstract the ISA differences (vec_vsx_ld vs vec_ld, dcbt TH hints vs basic dcbt). This is the first known instance of a 1-bit LLM running on big-endian hardware from 2003.

POWER8 Benchmarks

Hardware: IBM Power System S824 (8286-42A), 16c/128t POWER8, 512 GB DDR3
Config: 64 threads, numactl --interleave=all, OMP_PROC_BIND=spread

Scalar → VSX Speedup

Model	Params	pp128 (scalar)	pp128 (VSX)	Speedup
bitnet_b1_58-large	728M	21.48 t/s	211.48 t/s	9.8x
BitNet-b1.58-2B-4T	2.74B	8.04 t/s	73.03 t/s	9.1x
Llama3-8B-1.58	8.03B	2.60 t/s	27.39 t/s	10.5x

Full Results (VSX + dcbt)

Model	Size	pp128	pp256	pp512	tg32
bitnet_b1_58-large	257 MiB	209.38 t/s	176.67 t/s	134.10 t/s	24.02 t/s
BitNet-b1.58-2B-4T	1.71 GiB	71.95 t/s	64.98 t/s	52.67 t/s	11.99 t/s
Llama3-8B-1.58	3.58 GiB	26.98 t/s	25.06 t/s	21.70 t/s	5.63 t/s

Power Mac G5 Benchmarks (Big-Endian)

Hardware: Power Mac G5 Dual 2.0 GHz (PowerPC 970), 8 GB DDR2, Mac OS X 10.5.8
Compiler: GCC 10.5.0, -mcpu=970 -maltivec -Os

Model	Size	pp6	tg20
bitnet_b1_58-large	257 MiB	4.68 t/s	1.70 t/s

AltiVec kernel microbenchmark: The raw dot product kernel achieves 16.1x speedup over scalar (5.84 GMAC/s vs 0.36 GMAC/s on a 768×1536 matmul). End-to-end inference is limited by Amdahl's law: matmul is only 12-24% of total inference time. The remaining 76-88% is framework overhead (layernorm, softmax, RoPE, activation quantization, and 870 ggml_barrier() synchronizations per token).

Use -t 1 (single thread) for best G5 performance — barrier overhead on 870 graph nodes makes multi-threading counterproductive.

The G5 AltiVec kernels share the same code path as POWER8 VSX through 4 compatibility macros:

Macro	POWER8 (VSX)	G5 (AltiVec)
I2S_VEC_LD_UC	vec_vsx_ld (unaligned)	vec_ld (aligned)
I2S_VEC_LD_SC	vec_vsx_ld (signed char)	vec_ld (aligned)
I2S_DCBT_RESIDENT	dcbt TH=16 (L3 sticky)	dcbt (basic)
I2S_DCBT	dcbt TH=0 (transient)	dcbt (basic)

Vector constants use vec_splat_u8() (generates vspltisb instruction) instead of static const arrays, avoiding Mach-O alignment issues on old Darwin. No endian changes needed: vec_msum accumulates all 16 bytes into 4 int32 lanes, then hsum sums all 4 — the total is identical regardless of lane assignment order.

always_inline is critical on Mach-O: without it, every i2s_ppc_half call generates VRsave save/restore sequences (mfspr/mtspr, ~20 cycles each), which devastates performance in the inner loop.

Files Changed

• src/ggml-bitnet-mad.cpp — POWER8 VSX + G5 AltiVec I2_S kernels with compatibility macros
• include/gemm-config.h — PowerPC architecture detection (__VSX__, __ALTIVEC__, __powerpc__, __ppc__)
• include/bitnet-lut-kernels.h — PowerPC stub (LUT kernels are x86/ARM-specific)
• patches/g5-big-endian.patch — GGUF byte-swap for big-endian hosts
• patches/regex-ppc.h — POSIX regex wrapper for PPC big-endian
• patches/build_g5.sh — G5 AltiVec build script (-Os, llama-cli)
• README.md — Build instructions and benchmarks

Build

POWER8 (ppc64le)

cmake -B build -DCMAKE_C_FLAGS="-mcpu=power8 -mvsx" -DCMAKE_CXX_FLAGS="-mcpu=power8 -mvsx"
cmake --build build --config Release

Power Mac G5 (ppc64be)

./patches/build_g5.sh /usr/local/gcc-10/bin

Testing

• POWER8: Verified on IBM Power System S824 with bitnet_b1_58-large (700M), BitNet-b1.58-2B-4T (2B), and Llama3-8B-1.58 (8B). All models produce correct output.
• Power Mac G5: Verified on Power Mac G5 Dual 2.0 GHz with bitnet_b1_58-large (700M). Produces coherent English text output. AltiVec kernels verified active via microbenchmark (16x raw dot product speedup).
• POWER8 regression: Same code compiles and runs correctly with -mvsx — the compatibility macros resolve to the original VSX intrinsics on POWER8.

[Scottcjn]

@microsoft-github-policy-service agree

[Scottcjn]

i agree.

…

On Sat, Jan 31, 2026 at 11:51 AM microsoft-github-policy-service[bot] < ***@***.***> wrote: *microsoft-github-policy-service[bot]* left a comment (microsoft/BitNet#394) <#394 (comment)> @Scottcjn <https://github.com/Scottcjn> please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information. @microsoft-github-policy-service agree [company="{your company}"] Options: - (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer. @microsoft-github-policy-service agree - (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer. @microsoft-github-policy-service agree company="Microsoft" Contributor License Agreement Contribution License Agreement This Contribution License Agreement (*“Agreement”*) is agreed to by the party signing below (*“You”*), and conveys certain license rights to Microsoft Corporation and its affiliates (“Microsoft”) for Your contributions to Microsoft open source projects. This Agreement is effective as of the latest signature date below. 1. *Definitions*. *“Code”* means the computer software code, whether in human-readable or machine-executable form, that is delivered by You to Microsoft under this Agreement. *“Project”* means any of the projects owned or managed by Microsoft and offered under a license approved by the Open Source Initiative (www.opensource.org). *“Submit”* is the act of uploading, submitting, transmitting, or distributing code or other content to any Project, including but not limited to communication on electronic mailing lists, source code control systems, and issue tracking systems that are managed by, or on behalf of, the Project for the purpose of discussing and improving that Project, but excluding communication that is conspicuously marked or otherwise designated in writing by You as “Not a Submission.” *“Submission”* means the Code and any other copyrightable material Submitted by You, including any associated comments and documentation. 2. *Your Submission*. You must agree to the terms of this Agreement before making a Submission to any Project. This Agreement covers any and all Submissions that You, now or in the future (except as described in Section 4 below), Submit to any Project. 3. *Originality of Work*. You represent that each of Your Submissions is entirely Your original work. Should You wish to Submit materials that are not Your original work, You may Submit them separately to the Project if You (a) retain all copyright and license information that was in the materials as You received them, (b) in the description accompanying Your Submission, include the phrase “Submission containing materials of a third party:” followed by the names of the third party and any licenses or other restrictions of which You are aware, and (c) follow any other instructions in the Project’s written guidelines concerning Submissions. 4. *Your Employer*. References to “employer” in this Agreement include Your employer or anyone else for whom You are acting in making Your Submission, e.g. as a contractor, vendor, or agent. If Your Submission is made in the course of Your work for an employer or Your employer has intellectual property rights in Your Submission by contract or applicable law, You must secure permission from Your employer to make the Submission before signing this Agreement. In that case, the term “You” in this Agreement will refer to You and the employer collectively. If You change employers in the future and desire to Submit additional Submissions for the new employer, then You agree to sign a new Agreement and secure permission from the new employer before Submitting those Submissions. 5. *Licenses*. - *Copyright License*. You grant Microsoft, and those who receive the Submission directly or indirectly from Microsoft, a perpetual, worldwide, non-exclusive, royalty-free, irrevocable license in the Submission to reproduce, prepare derivative works of, publicly display, publicly perform, and distribute the Submission and such derivative works, and to sublicense any or all of the foregoing rights to third parties. - *Patent License*. You grant Microsoft, and those who receive the Submission directly or indirectly from Microsoft, a perpetual, worldwide, non-exclusive, royalty-free, irrevocable license under Your patent claims that are necessarily infringed by the Submission or the combination of the Submission with the Project to which it was Submitted to make, have made, use, offer to sell, sell and import or otherwise dispose of the Submission alone or with the Project. - *Other Rights Reserved*. Each party reserves all rights not expressly granted in this Agreement. No additional licenses or rights whatsoever (including, without limitation, any implied licenses) are granted by implication, exhaustion, estoppel or otherwise. 6. *Representations and Warranties*. You represent that You are legally entitled to grant the above licenses. You represent that each of Your Submissions is entirely Your original work (except as You may have disclosed under Section 3). You represent that You have secured permission from Your employer to make the Submission in cases where Your Submission is made in the course of Your work for Your employer or Your employer has intellectual property rights in Your Submission by contract or applicable law. If You are signing this Agreement on behalf of Your employer, You represent and warrant that You have the necessary authority to bind the listed employer to the obligations contained in this Agreement. You are not expected to provide support for Your Submission, unless You choose to do so. UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING, AND EXCEPT FOR THE WARRANTIES EXPRESSLY STATED IN SECTIONS 3, 4, AND 6, THE SUBMISSION PROVIDED UNDER THIS AGREEMENT IS PROVIDED WITHOUT WARRANTY OF ANY KIND, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY OF NONINFRINGEMENT, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. 7. *Notice to Microsoft*. You agree to notify Microsoft in writing of any facts or circumstances of which You later become aware that would make Your representations in this Agreement inaccurate in any respect. 8. *Information about Submissions*. You agree that contributions to Projects and information about contributions may be maintained indefinitely and disclosed publicly, including Your name and other information that You submit with Your Submission. 9. *Governing Law/Jurisdiction*. This Agreement is governed by the laws of the State of Washington, and the parties consent to exclusive jurisdiction and venue in the federal courts sitting in King County, Washington, unless no federal subject matter jurisdiction exists, in which case the parties consent to exclusive jurisdiction and venue in the Superior Court of King County, Washington. The parties waive all defenses of lack of personal jurisdiction and forum non-conveniens. 10. *Entire Agreement/Assignment*. This Agreement is the entire agreement between the parties, and supersedes any and all prior agreements, understandings or communications, written or oral, between the parties relating to the subject matter hereof. This Agreement may be assigned by Microsoft. — Reply to this email directly, view it on GitHub <#394 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A45PBVCUG2SO3PN7CR2PBPT4JTTQXAVCNFSM6AAAAACTRJS6AGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQMRYHEYTQMBWGU> . You are receiving this because you were mentioned.Message ID: ***@***.***>