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Convert -Ctarget-cpu into a target-modifier for NVPTX #150732

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Convert -Ctarget-cpu into a target-modifier for NVPTX #150732

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@kulst kulst commented Jan 6, 2026 •
edited by RalfJung
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For NVPTX -Ctarget-cpu needs to be the same in all crates linked together into a binary.
This PR converts -Ctarget-cpu into a target-modifier. However, the agreement about it is only enforced when compiling for specific targets based on a flag in the target specification. This makes -Ctarget-cpu effectively a target-modifier only for NVPTX, for which this flag is set.

Previous discussions about this topic can be found here and here.

I also created a Zulip discussion.

I am unsure if a MCP is needed before proceeding. If you think so please let me know.

Creating target-modifiers for NVPTX target-features is to be done in a follow-up.

cc @kjetilkjeka as target maintainer for NVPTX
cc @Flakebi as you might need the same feature for amdgpu
cc @RalfJung you were very involved in the discussions so far

Target modifier tracking issue: #136966

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 6, 2026
@kulst kulst changed the title Making -Ctarget-cpu a target-modifier on NVPTX Make -Ctarget-cpu a target-modifier on NVPTX Jan 6, 2026
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kulst commented Jan 6, 2026

@rustbot label: +O-NVPTX

@rustbot rustbot added the O-NVPTX Target: the NVPTX LLVM backend for running rust on GPUs, https://llvm.org/docs/NVPTXUsage.html label Jan 6, 2026
@kulst kulst changed the title Make -Ctarget-cpu a target-modifier on NVPTX Convert -Ctarget-cpu into a target-modifier on NVPTX Jan 7, 2026
@kulst kulst changed the title Convert -Ctarget-cpu into a target-modifier on NVPTX Convert -Ctarget-cpu into a target-modifier for NVPTX Jan 7, 2026
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Convert -Ctarget-cpu into a target-modifier for NVPTX
a8f5e96 
For NVPTX `-Ctarget-cpu` needs to be the same in all crates linked together
into a binary.
This converts `-Ctarget-cpu` into a target-modifier. However, the agreement
about it is only enforced when compiling for specific targets based on a flag
in the target specification. This makes `-Ctarget-cpu` effectively a
target-modifier only for NVPTX for which this flag is set.
@kulst kulst force-pushed the cpu_is_target_modifier branch from 7164067 to a8f5e96 Compare January 7, 2026 21:26
@kulst kulst marked this pull request as ready for review January 7, 2026 21:34
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rustbot commented Jan 7, 2026

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

These commits modify compiler targets.
(See the Target Tier Policy.)

@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 7, 2026
@rustbot rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 7, 2026
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rustbot commented Jan 7, 2026

r? @nnethercote

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kulst commented Jan 7, 2026 •
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r? @bjorn3 as you suggested this change.

@rustbot rustbot assigned bjorn3 and unassigned nnethercote Jan 7, 2026
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bjorn3 reviewed Jan 8, 2026
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compiler/rustc_target/src/spec/targets/nvptx64_nvidia_cuda.rs
// With `ptx-linker` approach, it can be later overridden via link flags.
cpu: "sm_30".into(),
// The target cpu for NVPTX must be the same for a complete binary.
cpu_is_target_modifier: true,
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I believe AVR has the same requirement and probably amdgcn too. Also why does nvptx not have need_explicit_cpu set? Would we ever need to set need_explicit_cpu without cpu_is_target_modifier and vice versa?

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kjetilkjeka commented Jan 8, 2026

It works out really nice with -CTarget-cpu being able to be a target modifier.

It would be nice to know what the plan was with the ptx isa version as well. I asked that question here

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RalfJung commented Jan 8, 2026

I can't vouch for the implementation, but the approach sounds good. :)

Cc @Darksonn

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