[libc] fix type generic stdc_leading_zeros for GCC

[nickdesaulniers]

GCC does not support _Generic in C++ mode. Use inline function definitions
with function overloading in __cplusplus mode.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

Changes

GCC does not support _Generic in C++ mode. Use inline function definitions
with function overloading in __cplusplus mode.

---

Full diff: https://github.com/llvm/llvm-project/pull/79917.diff

2 Files Affected:

• (modified) libc/include/llvm-libc-macros/stdbit-macros.h (+18)
• (modified) libc/include/stdbit.h.def (+2-1)

diff --git a/libc/include/llvm-libc-macros/stdbit-macros.h b/libc/include/llvm-libc-macros/stdbit-macros.h
index febe95fe0a1e3c..da0fb1a578f139 100644
--- a/libc/include/llvm-libc-macros/stdbit-macros.h
+++ b/libc/include/llvm-libc-macros/stdbit-macros.h
@@ -9,6 +9,23 @@
 #ifndef __LLVM_LIBC_MACROS_STDBIT_MACROS_H
 #define __LLVM_LIBC_MACROS_STDBIT_MACROS_H
 
+#ifdef __cplusplus
+inline unsigned char stdc_leading_zeros(unsigned char x) {
+  return stdc_leading_zeros_uc(x);
+}
+inline unsigned short stdc_leading_zeros(unsigned short x) {
+  return stdc_leading_zeros_us(x);
+}
+inline unsigned stdc_leading_zeros(unsigned x) {
+  return stdc_leading_zeros_ui(x);
+}
+inline unsigned long stdc_leading_zeros(unsigned long x) {
+  return stdc_leading_zeros_ul(x);
+}
+inline unsigned long long stdc_leading_zeros(unsigned long long x) {
+  return stdc_leading_zeros_ull(x);
+}
+#else
 #define stdc_leading_zeros(x)                                                  \
   _Generic((x),                                                                \
       unsigned char: stdc_leading_zeros_uc,                                    \
@@ -16,5 +33,6 @@
       unsigned: stdc_leading_zeros_ui,                                         \
       unsigned long: stdc_leading_zeros_ul,                                    \
       unsigned long long: stdc_leading_zeros_ull)(x)
+#endif // __cplusplus
 
 #endif // __LLVM_LIBC_MACROS_STDBIT_MACROS_H
diff --git a/libc/include/stdbit.h.def b/libc/include/stdbit.h.def
index cb79ac1caf049f..c5a77329fbfe74 100644
--- a/libc/include/stdbit.h.def
+++ b/libc/include/stdbit.h.def
@@ -10,8 +10,9 @@
 #define LLVM_LIBC_STDBIT_H
 
 #include <__llvm-libc-common.h>
-#include <llvm-libc-macros/stdbit-macros.h>
 
 %%public_api()
 
+#include <llvm-libc-macros/stdbit-macros.h>
+
 #endif // LLVM_LIBC_STDBIT_H

[jplehr]

I think this broke the AMDGPU libc bot. https://lab.llvm.org/staging/#/builders/11/builds/1282
@jhuber6 can you take a look?

[nickdesaulniers]

Fixed in 0129ff1. Sorry about that!

@jhuber6 I think we can simplify the condition further to just:

if(libc.include.stdbit IN_LIST TARGET_PUBLIC_HEADERS)

(I'm guessing that TARGET_PUBLIC_HEADERS is empty when LLVM_LIBC_FULL_BUILD is not set?)

[nickdesaulniers]

Also, we can probably add this macro to the gpu headers file.

[jhuber6]

Also, we can probably add this macro to the gpu headers file.

It's possible, I think I tried and it had some other problem so I just ignored it for now. We can reevaluate later.

[jhuber6]

Fixed in 0129ff1. Sorry about that!

@jhuber6 I think we can simplify the condition further to just:

if(libc.include.stdbit IN_LIST TARGET_PUBLIC_HEADERS)

(I'm guessing that TARGET_PUBLIC_HEADERS is empty when LLVM_LIBC_FULL_BUILD is not set?)

Don't think so, but we could alter the logic to do that if it's the desire behavior.

diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
index 6d385849b6a6..66d8f702d420 100644
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -314,7 +314,7 @@ else()
 endif()
 
 # Check headers.txt
-if(EXISTS "${LIBC_CONFIG_PATH}/headers.txt")
+if(EXISTS "${LIBC_CONFIG_PATH}/headers.txt" AND LLVM_LIBC_FULL_BUILD)
     include("${LIBC_CONFIG_PATH}/headers.txt")
 elseif(LLVM_LIBC_FULL_BUILD)
   message(FATAL_ERROR "${LIBC_CONFIG_PATH}/headers.txt file not found and fullbuild requested.")

Or something. might break something else, who knows.