[BugFix][Graph] fix: handle empty sym_shape_indices in PiecewiseBackend.

[chaunceyjiang]

Purpose

FIx #39341

Test Plan

generating_prompts = [prefix + prompt for prompt in prompts]
llm = LLM(
    model="/mnt/data4/models/Qwen/Qwen3-0.6B",
    max_model_len=512,
    max_num_batched_tokens=1,
)
sampling_params = SamplingParams(temperature=0, top_p=0.95, max_tokens=10)

outputs = llm.generate(generating_prompts, sampling_params)
print(outputs)

Test Result

[RequestOutput(request_id=0, prompt="You are an expert school principal, skilled in effectively managing faculty and staff. Draft 10-15 questions for a potential first grade Head Teacher for my K-12, all-girls', independent school that emphasizes community, joyful discovery, and life-long learning. The candidate is coming in for a first-round panel interview for a 8th grade Math teaching role. They have 5 years of previous teaching experience as an assistant teacher at a co-ed, public school with experience in middle school math teaching. Based on these information, fulfill the following paragraph: Hello, my name is", prompt_token_ids=[2610, 525, 458, 6203, 2906, 12435, 11, 25530, 304, 13444, 18150, 21564, 323, 5570, 13, 28564, 220, 16, 15, 12, 16, 20, 4755, 369, 264, 4650, 1156, 11972, 11203, 29069, 369, 847, 730, 12, 16, 17, 11, 678, 62870, 82, 516, 9489, 2906, 429, 65059, 3942, 11, 82112, 18335, 11, 323, 2272, 23791, 6832, 13, 576, 9144, 374, 5001, 304, 369, 264, 1156, 21015, 6945, 7128, 369, 264, 220, 23, 339, 11972, 4149, 12629, 3476, 13, 2379, 614, 220, 20, 1635, 315, 3681, 12629, 3139, 438, 458, 17847, 11079, 518, 264, 1062, 34435, 11, 584, 2906, 448, 3139, 304, 6149, 2906, 6888, 12629, 13, 20205, 389, 1493, 1995, 11, 20423, 279, 2701, 14311, 25, 21927, 11, 847, 829, 374], encoder_prompt=None, encoder_prompt_token_ids=None, prompt_logprobs=None, outputs=[CompletionOutput(index=0, text=' [Name], and I am a [Position]', token_ids=[508, 675, 1125, 323, 358, 1079, 264, 508, 3812, 60], routed_experts=None, cumulative_logprob=None, logprobs=None, finish_reason=length, stop_reason=None)], finished=True, metrics=None, lora_request=None, num_cached_tokens=0), RequestOutput(request_id=1, prompt="You are an expert school principal, skilled in effectively managing faculty and staff. Draft 10-15 questions for a potential first grade Head Teacher for my K-12, all-girls', independent school that emphasizes community, joyful discovery, and life-long learning. The candidate is coming in for a first-round panel interview for a 8th grade Math teaching role. They have 5 years of previous teaching experience as an assistant teacher at a co-ed, public school with experience in middle school math teaching. Based on these information, fulfill the following paragraph: The president of the United States is", prompt_token_ids=[2610, 525, 458, 6203, 2906, 12435, 11, 25530, 304, 13444, 18150, 21564, 323, 5570, 13, 28564, 220, 16, 15, 12, 16, 20, 4755, 369, 264, 4650, 1156, 11972, 11203, 29069, 369, 847, 730, 12, 16, 17, 11, 678, 62870, 82, 516, 9489, 2906, 429, 65059, 3942, 11, 82112, 18335, 11, 323, 2272, 23791, 6832, 13, 576, 9144, 374, 5001, 304, 369, 264, 1156, 21015, 6945, 7128, 369, 264, 220, 23, 339, 11972, 4149, 12629, 3476, 13, 2379, 614, 220, 20, 1635, 315, 3681, 12629, 3139, 438, 458, 17847, 11079, 518, 264, 1062, 34435, 11, 584, 2906, 448, 3139, 304, 6149, 2906, 6888, 12629, 13, 20205, 389, 1493, 1995, 11, 20423, 279, 2701, 14311, 25, 576, 4767, 315, 279, 3639, 4180, 374], encoder_prompt=None, encoder_prompt_token_ids=None, prompt_logprobs=None, outputs=[CompletionOutput(index=0, text=' a woman, and the president is a woman.', token_ids=[264, 5220, 11, 323, 279, 4767, 374, 264, 5220, 13], routed_experts=None, cumulative_logprob=None, logprobs=None, finish_reason=length, stop_reason=None)], finished=True, metrics=None, lora_request=None, num_cached_tokens=112), RequestOutput(request_id=2, prompt="You are an expert school principal, skilled in effectively managing faculty and staff. Draft 10-15 questions for a potential first grade Head Teacher for my K-12, all-girls', independent school that emphasizes community, joyful discovery, and life-long learning. The candidate is coming in for a first-round panel interview for a 8th grade Math teaching role. They have 5 years of previous teaching experience as an assistant teacher at a co-ed, public school with experience in middle school math teaching. Based on these information, fulfill the following paragraph: The capital of France is", prompt_token_ids=[2610, 525, 458, 6203, 2906, 12435, 11, 25530, 304, 13444, 18150, 21564, 323, 5570, 13, 28564, 220, 16, 15, 12, 16, 20, 4755, 369, 264, 4650, 1156, 11972, 11203, 29069, 369, 847, 730, 12, 16, 17, 11, 678, 62870, 82, 516, 9489, 2906, 429, 65059, 3942, 11, 82112, 18335, 11, 323, 2272, 23791, 6832, 13, 576, 9144, 374, 5001, 304, 369, 264, 1156, 21015, 6945, 7128, 369, 264, 220, 23, 339, 11972, 4149, 12629, 3476, 13, 2379, 614, 220, 20, 1635, 315, 3681, 12629, 3139, 438, 458, 17847, 11079, 518, 264, 1062, 34435, 11, 584, 2906, 448, 3139, 304, 6149, 2906, 6888, 12629, 13, 20205, 389, 1493, 1995, 11, 20423, 279, 2701, 14311, 25, 576, 6722, 315, 9625, 374], encoder_prompt=None, encoder_prompt_token_ids=None, prompt_logprobs=None, outputs=[CompletionOutput(index=0, text=' Paris, and the capital of Italy is Rome.', token_ids=[12095, 11, 323, 279, 6722, 315, 15344, 374, 21718, 13], routed_experts=None, cumulative_logprob=None, logprobs=None, finish_reason=length, stop_reason=None)], finished=True, metrics=None, lora_request=None, num_cached_tokens=112), RequestOutput(request_id=3, prompt="You are an expert school principal, skilled in effectively managing faculty and staff. Draft 10-15 questions for a potential first grade Head Teacher for my K-12, all-girls', independent school that emphasizes community, joyful discovery, and life-long learning. The candidate is coming in for a first-round panel interview for a 8th grade Math teaching role. They have 5 years of previous teaching experience as an assistant teacher at a co-ed, public school with experience in middle school math teaching. Based on these information, fulfill the following paragraph: The future of AI is", prompt_token_ids=[2610, 525, 458, 6203, 2906, 12435, 11, 25530, 304, 13444, 18150, 21564, 323, 5570, 13, 28564, 220, 16, 15, 12, 16, 20, 4755, 369, 264, 4650, 1156, 11972, 11203, 29069, 369, 847, 730, 12, 16, 17, 11, 678, 62870, 82, 516, 9489, 2906, 429, 65059, 3942, 11, 82112, 18335, 11, 323, 2272, 23791, 6832, 13, 576, 9144, 374, 5001, 304, 369, 264, 1156, 21015, 6945, 7128, 369, 264, 220, 23, 339, 11972, 4149, 12629, 3476, 13, 2379, 614, 220, 20, 1635, 315, 3681, 12629, 3139, 438, 458, 17847, 11079, 518, 264, 1062, 34435, 11, 584, 2906, 448, 3139, 304, 6149, 2906, 6888, 12629, 13, 20205, 389, 1493, 1995, 11, 20423, 279, 2701, 14311, 25, 576, 3853, 315, 15235, 374], encoder_prompt=None, encoder_prompt_token_ids=None, prompt_logprobs=None, outputs=[CompletionOutput(index=0, text=' not just about the technology itself, but about how', token_ids=[537, 1101, 911, 279, 5440, 5086, 11, 714, 911, 1246], routed_experts=None, cumulative_logprob=None, logprobs=None, finish_reason=length, stop_reason=None)], finished=True, metrics=None, lora_request=None, num_cached_tokens=112)]

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request modifies the __call__ method in vllm/compilation/piecewise_backend.py to introduce conditional logic. It now differentiates between dynamic and static input shapes based on self.sym_shape_indices. For static shapes, the method directly retrieves the first compiled range_entry from self.range_entries rather than attempting to derive a runtime shape from the arguments. This change improves the backend's flexibility to handle both dynamic and static input shape scenarios. There is no feedback to provide.

[chaunceyjiang]

/cc @ProExpertProg @zou3519 PTAL.

[DarkLight1337]

Can you add a regression test?

[chaunceyjiang]

Can you add a regression test?

Done.

[youkaichao]

cc @zou3519 is it possible to ask Dynamo just not to specialize on the size 1? I feel ideally this should be fixed by Dynamo.

[ProExpertProg]

cc @zou3519 is it possible to ask Dynamo just not to specialize on the size 1? I feel ideally this should be fixed by Dynamo.

I think this is possible with unbacked ints. @laithsakka is working on this I believe - faith anything to add?

[ProExpertProg]

Richard is away this week but it'd be good to wait for @laithsakka or @BoyuanFeng to take a look before merging