[libc-commits] [libc] [libc] fix stdbit include test when not all entrypoints are available (PR #80323)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers created https://github.com/llvm/llvm-project/pull/80323

The intent of the test is to check that:
1. The type generic macros are defined.
2. Those macros dispatch to the correct underlying function.

The issue is that when new functionality is added to our stdbit.h without
rolling out the new entrypoint to all targets, this test breaks because our
generated stdbit.h will not contain declarations for the underlying function.
In that case, we should just declare the underlying functions first before
including our generated stdbit.h which just contains declarations. A definition
is a declaration, but redeclarations must match, hence the additions of
noexcept and extern "C".


>From e55fb44118c08bf68b9b77fca72d31d7459ea8dd Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Thu, 1 Feb 2024 10:51:29 -0800
Subject: [PATCH] [libc] fix stdbit include test when not all entrypoints are
 available

The intent of the test is to check that:
1. The type generic macros are defined.
2. Those macros dispatch to the correct underlying function.

The issue is that when new functionality is added to our stdbit.h without
rolling out the new entrypoint to all targets, this test breaks because our
generated stdbit.h will not contain declarations for the underlying function.
In that case, we should just declare the underlying functions first before
including our generated stdbit.h which just contains declarations. A definition
is a declaration, but redeclarations must match, hence the additions of
noexcept and extern "C".
---
 libc/test/include/stdbit_test.cpp | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/libc/test/include/stdbit_test.cpp b/libc/test/include/stdbit_test.cpp
index 61e241c8f68a9..4e8498621b683 100644
--- a/libc/test/include/stdbit_test.cpp
+++ b/libc/test/include/stdbit_test.cpp
@@ -8,8 +8,6 @@
 
 #include "test/UnitTest/Test.h"
 
-#include <stdbit.h>
-
 /*
  * The intent of this test is validate that:
  * 1. We provide the definition of the various type generic macros of stdbit.h.
@@ -18,16 +16,24 @@
  * do not contain non-namespaced symbols.
  */
 
-unsigned char stdc_leading_zeros_uc(unsigned char) { return 0xAA; }
-unsigned short stdc_leading_zeros_us(unsigned short) { return 0xAB; }
-unsigned stdc_leading_zeros_ui(unsigned) { return 0xAC; }
-unsigned long stdc_leading_zeros_ul(unsigned long) { return 0xAD; }
-unsigned long long stdc_leading_zeros_ull(unsigned long long) { return 0xAF; }
-unsigned char stdc_leading_ones_uc(unsigned char) { return 0xBA; }
-unsigned short stdc_leading_ones_us(unsigned short) { return 0xBB; }
-unsigned stdc_leading_ones_ui(unsigned) { return 0xBC; }
-unsigned long stdc_leading_ones_ul(unsigned long) { return 0xBD; }
-unsigned long long stdc_leading_ones_ull(unsigned long long) { return 0xBF; }
+/*
+ * Declare these BEFORE including stdbit.h so that this test may still be run
+ * even if a given target doesn't yet have these individual entrypoints enabled.
+ */
+extern "C" {
+unsigned char stdc_leading_zeros_uc(unsigned char) noexcept { return 0xAA; }
+unsigned short stdc_leading_zeros_us(unsigned short) noexcept { return 0xAB; }
+unsigned stdc_leading_zeros_ui(unsigned) noexcept { return 0xAC; }
+unsigned long stdc_leading_zeros_ul(unsigned long) noexcept { return 0xAD; }
+unsigned long long stdc_leading_zeros_ull(unsigned long long) noexcept { return 0xAF; }
+unsigned char stdc_leading_ones_uc(unsigned char) noexcept { return 0xBA; }
+unsigned short stdc_leading_ones_us(unsigned short) noexcept { return 0xBB; }
+unsigned stdc_leading_ones_ui(unsigned) noexcept { return 0xBC; }
+unsigned long stdc_leading_ones_ul(unsigned long) noexcept { return 0xBD; }
+unsigned long long stdc_leading_ones_ull(unsigned long long) noexcept { return 0xBF; }
+}
+
+#include <stdbit.h>
 
 TEST(LlvmLibcStdbitTest, TypeGenericMacroLeadingZeros) {
   EXPECT_EQ(stdc_leading_zeros(static_cast<unsigned char>(0U)),