[BPF] make __BPF_FEATURE_MAY_GOTO available for cpuv1

[eddyz87]

For some reason __BPF_FEATURE_MAY_GOTO is available for CPUs v{2,3,4} but is not available for CPU v1. This limitation is arbitrary:

• the instruction is never produced by LLVM backend;
• on Linux Kernel side this instruction is available in kernels that also support CPUv4.

Hence, it is more consistent to either always allow __BPF_FEATURE_MAY_GOTO or only allow it for CPUv4.

[eddyz87]

@yonghong-song, @4ast,

As a cleanup, Yonghong suggested moving __BPF_FEATURE_MAY_GOTO macro so that it is available for CPUv1.
(Although, I think it's better to move both __BPF_FEATURE_MAY_GOTO and __BPF_FEATURE_ADDR_SPACE_CAST down to CPUv4, as this would be more consistent with kernel features support).

Wdyt?

[llvmbot]

@llvm/pr-subscribers-clang

Author: None (eddyz87)

Changes

For some reason __BPF_FEATURE_MAY_GOTO is available for CPUs v{2,3,4} but is not available for CPU v1. This limitation is arbitrary:

• the instruction is never produced by LLVM backend;
• on Linux Kernel side this instruction is available in kernels that also support CPUv4.

Hence, it is more consistent to either always allow __BPF_FEATURE_MAY_GOTO or only allow it for CPUv4.

---

Full diff: https://github.com/llvm/llvm-project/pull/108071.diff

1 Files Affected:

• (modified) clang/lib/Basic/Targets/BPF.cpp (+1-1)

diff --git a/clang/lib/Basic/Targets/BPF.cpp b/clang/lib/Basic/Targets/BPF.cpp
index a94ceee5a6a5e7..931f407ecb0d7e 100644
--- a/clang/lib/Basic/Targets/BPF.cpp
+++ b/clang/lib/Basic/Targets/BPF.cpp
@@ -37,6 +37,7 @@ void BPFTargetInfo::getTargetDefines(const LangOptions &Opts,
   }
 
   Builder.defineMacro("__BPF_FEATURE_ADDR_SPACE_CAST");
+  Builder.defineMacro("__BPF_FEATURE_MAY_GOTO");
 
   if (CPU.empty())
     CPU = "v3";
@@ -48,7 +49,6 @@ void BPFTargetInfo::getTargetDefines(const LangOptions &Opts,
 
   std::string CpuVerNumStr = CPU.substr(1);
   Builder.defineMacro("__BPF_CPU_VERSION__", CpuVerNumStr);
-  Builder.defineMacro("__BPF_FEATURE_MAY_GOTO");
 
   int CpuVerNum = std::stoi(CpuVerNumStr);
   if (CpuVerNum >= 2)

[4ast]

@yonghong-song, @4ast,

As a cleanup, Yonghong suggested moving __BPF_FEATURE_MAY_GOTO macro so that it is available for CPUv1. (Although, I think it's better to move both __BPF_FEATURE_MAY_GOTO and __BPF_FEATURE_ADDR_SPACE_CAST down to CPUv4, as this would be more consistent with kernel features support).

Wdyt?

why force users to specify -mcpu=v4 ?
may_goto and addr_cast are fine with v1.

[eddyz87]

@4ast,

why force users to specify -mcpu=v4 ? may_goto and addr_cast are fine with v1.

Ok, then this is exactly what this pull request does (allows may_goto for v1).

[eddyz87]

@4ast, ping