[clang] [Clang][BPF] Add __bpf_cpu_version__ macro (PR #71856)

[via cfe-commits]

https://github.com/yonghong-song created https://github.com/llvm/llvm-project/pull/71856

Sometimes bpf developer might want to develop different codes based on particular cpu versioins. For example, cpu v1/v2/v3 branch target is 16bit while cpu v4 branch target is 32bit, thus cpu v4 allows more aggressive loop unrolling than cpu v1/v2/v3 (see [1] for a kernel selftest failure due to this). We would like to maintain aggressive loop unrolling for cpu v4 while limit loop unrolling for earlier cpu versions. Another example, signed divide also only available with cpu v4.

Actually, adding cpu specific macros are fairly common in llvm. For example, x86 has maco like 'i486', '__pentium_mmx__', etc. AArch64 has '__ARM_NEON', '__ARM_FEATURE_SVE', etc.

This patch added __bpf_cpu_version__ macro. Current possible values are 0/1/2/3/4. The following are the -mcpu=... to __bpf_cpu_version__ mapping:
   cpu                  __bpf_cpu_version__
   no -mcpu=<...>       1
   -mcpu=v1             1
   -mcpu=v2             2
   -mcpu=v3             3
   -mcpu=v4             4
   -mcpu=generic        1
   -mcpu=probe          0

 [1] https://lore.kernel.org/bpf/3e3a8a30-dde0-43a1-981e-2274962780ef@linux.dev/

>From 7780450832a0d2fd7310973a964890c2cdf71c2c Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Thu, 9 Nov 2023 11:15:09 -0800
Subject: [PATCH] [Clang][BPF] Add __bpf_cpu_version__ macro

Sometimes bpf developer might want to develop different codes
based on particular cpu versioins. For example, cpu v1/v2/v3
branch target is 16bit while cpu v4 branch target is 32bit,
thus cpu v4 allows more aggressive loop unrolling than cpu v1/v2/v3
(see [1] for a kernel selftest failure due to this).
We would like to maintain aggressive loop unrolling for cpu v4
while limit loop unrolling for earlier cpu versions.
Another example, signed divide also only available with cpu v4.

Actually, adding cpu specific macros are fairly common
in llvm. For example, x86 has maco like 'i486', '__pentium_mmx__', etc.
AArch64 has '__ARM_NEON', '__ARM_FEATURE_SVE', etc.

This patch added __bpf_cpu_version__ macro. Current possible values
are 0/1/2/3/4. The following are the -mcpu=... to __bpf_cpu_version__
mapping:
   cpu                  __bpf_cpu_version__
   no -mcpu=<...>       1
   -mcpu=v1             1
   -mcpu=v2             2
   -mcpu=v3             3
   -mcpu=v4             4
   -mcpu=generic        1
   -mcpu=probe          0

 [1] https://lore.kernel.org/bpf/3e3a8a30-dde0-43a1-981e-2274962780ef@linux.dev/
---
 clang/lib/Basic/Targets/BPF.cpp               |  8 ++++
 .../test/Preprocessor/bpf-predefined-macros.c | 37 ++++++++++++++++++-
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Basic/Targets/BPF.cpp b/clang/lib/Basic/Targets/BPF.cpp
index d6288d2e0d0e176..a61e279f395ea31 100644
--- a/clang/lib/Basic/Targets/BPF.cpp
+++ b/clang/lib/Basic/Targets/BPF.cpp
@@ -29,6 +29,14 @@ void BPFTargetInfo::getTargetDefines(const LangOptions &Opts,
                                      MacroBuilder &Builder) const {
   Builder.defineMacro("__bpf__");
   Builder.defineMacro("__BPF__");
+
+  std::string CPU = getTargetOpts().CPU;
+  if (CPU == "probe")
+    Builder.defineMacro("__bpf_cpu_version__", "0");
+  else if (CPU.empty() || CPU == "generic")
+    Builder.defineMacro("__bpf_cpu_version__", "1");
+  else
+    Builder.defineMacro("__bpf_cpu_version__", CPU.substr(1));
 }
 
 static constexpr llvm::StringLiteral ValidCPUNames[] = {"generic", "v1", "v2",
diff --git a/clang/test/Preprocessor/bpf-predefined-macros.c b/clang/test/Preprocessor/bpf-predefined-macros.c
index bcb985f95426622..364f4cc3f4d0c3c 100644
--- a/clang/test/Preprocessor/bpf-predefined-macros.c
+++ b/clang/test/Preprocessor/bpf-predefined-macros.c
@@ -1,5 +1,11 @@
-// RUN: %clang -E -target bpfel -x c -o - %s | FileCheck %s
-// RUN: %clang -E -target bpfeb -x c -o - %s | FileCheck %s
+// RUN: %clang -E -target bpfel -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_NO %s
+// RUN: %clang -E -target bpfeb -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_NO %s
+// RUN: %clang -E -target bpfel -mcpu=v1 -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_V1 %s
+// RUN: %clang -E -target bpfel -mcpu=v2 -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_V2 %s
+// RUN: %clang -E -target bpfel -mcpu=v3 -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_V3 %s
+// RUN: %clang -E -target bpfel -mcpu=v4 -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_V4 %s
+// RUN: %clang -E -target bpfel -mcpu=generic -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_GENERIC %s
+// RUN: %clang -E -target bpfel -mcpu=probe -x c -o - %s | FileCheck -check-prefix=CHECK -check-prefix=CPU_PROBE %s
 
 #ifdef __bpf__
 int b;
@@ -10,7 +16,34 @@ int c;
 #ifdef bpf
 int d;
 #endif
+#ifdef __bpf_cpu_version__
+int e;
+#endif
+#if __bpf_cpu_version__ == 0
+int f;
+#endif
+#if __bpf_cpu_version__ == 1
+int g;
+#endif
+#if __bpf_cpu_version__ == 2
+int h;
+#endif
+#if __bpf_cpu_version__ == 3
+int i;
+#endif
+#if __bpf_cpu_version__ == 4
+int j;
+#endif
 
 // CHECK: int b;
 // CHECK: int c;
 // CHECK-NOT: int d;
+// CHECK: int e;
+
+// CPU_NO: int g;
+// CPU_V1: int g;
+// CPU_V2: int h;
+// CPU_V3: int i;
+// CPU_V4: int j;
+// CPU_GENERIC: int g;
+// CPU_PROBE: int f;