[libc] Attempt to enable missing string functions for offloading

[jhuber6]

Summary:
We previously had to disable these string functions because they were
not compatible with the definitions coming from the GNU / host
environment. The GPU, when exporting its declarations, has a very
difficult requirement that it be compatible with the host environment as
both sides of the compilation need to agree on definitions and what's
present.

For string.h, some functions have different defintions for C and C++.
GNU would optionally provide the C++ definitions directly in the
string.h file. This patch attempts to hack around this by tricking the
GNU headers into thinking that the clang used it too old to support
their special definition handling. The blast radius from this hack
should be somewhat minimal, only changing C++ behaviour for a few
functions. The only noticable change is that we lose const error
checking for a small handful of functions if the user includes
string.h directly in C++ mode instead of .

[llvmbot]

@llvm/pr-subscribers-backend-x86
@llvm/pr-subscribers-libc

@llvm/pr-subscribers-clang

Changes

Summary:
We previously had to disable these string functions because they were
not compatible with the definitions coming from the GNU / host
environment. The GPU, when exporting its declarations, has a very
difficult requirement that it be compatible with the host environment as
both sides of the compilation need to agree on definitions and what's
present.

For string.h, some functions have different defintions for C and C++.
GNU would optionally provide the C++ definitions directly in the
string.h file. This patch attempts to hack around this by tricking the
GNU headers into thinking that the clang used it too old to support
their special definition handling. The blast radius from this hack
should be somewhat minimal, only changing C++ behaviour for a few
functions. The only noticable change is that we lose const error
checking for a small handful of functions if the user includes
string.h directly in C++ mode instead of <cstring>.

---

Full diff: https://github.com/llvm/llvm-project/pull/67869.diff

3 Files Affected:

• (modified) clang/lib/Headers/llvm_libc_wrappers/string.h (+24-3)
• (modified) libc/config/gpu/entrypoints.txt (+16)
• (modified) libc/docs/gpu/support.rst (+19-8)

diff --git a/clang/lib/Headers/llvm_libc_wrappers/string.h b/clang/lib/Headers/llvm_libc_wrappers/string.h
index 027c415c1d0f8f3..0fa6d81fda36b61 100644
--- a/clang/lib/Headers/llvm_libc_wrappers/string.h
+++ b/clang/lib/Headers/llvm_libc_wrappers/string.h
@@ -13,11 +13,32 @@
 #error "This file is for GPU offloading compilation only"
 #endif
 
-// FIXME: The GNU headers provide C++ standard compliant headers when in C++
-// mode and the LLVM libc does not. We cannot enable memchr, strchr, strchrnul,
-// strpbrk, strrchr, strstr, or strcasestr until this is addressed.
+// The GNU headers provide C++ standard compliant headers when in C++ mode and
+// the LLVM libc does not. We need to perform a pretty nasty hack to trick the
+// GNU headers into emitting the C compatible definitions so we can use them.
+#if defined(__cplusplus) && defined(__GLIBC__)
+
+// We need to make sure that the GNU C library has done its setup before we mess
+// with the expected macro values.
+#if !defined(__GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION) &&               \
+    __has_include(<bits/libc-header-start.h>)
+#define __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION
+#include <bits/libc-header-start.h>
+#endif
+
+// Trick the GNU headers into thinking that this clang is too old for the C++
+// definitions.
+#pragma push_macro("__clang_major__")
+#define __clang_major__ 3
+#endif
+
 #include_next <string.h>
 
+// Resore the original macros if they were changed.
+#if defined(__cplusplus) && defined(__GLIBC__)
+#pragma pop_macro("__clang_major__")
+#endif
+
 #if __has_include(<llvm-libc-decls/string.h>)
 
 #if defined(__HIP__) || defined(__CUDA__)
diff --git a/libc/config/gpu/entrypoints.txt b/libc/config/gpu/entrypoints.txt
index ad68216a76b9429..dcaca15b500cbe9 100644
--- a/libc/config/gpu/entrypoints.txt
+++ b/libc/config/gpu/entrypoints.txt
@@ -22,21 +22,31 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # string.h entrypoints
     libc.src.string.bcmp
+    libc.src.string.bcopy
     libc.src.string.bzero
+    libc.src.string.index
     libc.src.string.memccpy
+    libc.src.string.memchr
     libc.src.string.memcmp
     libc.src.string.memcpy
     libc.src.string.memmem
     libc.src.string.memmove
     libc.src.string.mempcpy
+    libc.src.string.memrchr
     libc.src.string.memset
+    libc.src.string.rindex
     libc.src.string.stpcpy
     libc.src.string.stpncpy
     libc.src.string.strcasecmp
+    libc.src.string.strcasestr
     libc.src.string.strcat
+    libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
+    libc.src.string.strcoll
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strdup
     libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
@@ -44,10 +54,16 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strncat
     libc.src.string.strncmp
     libc.src.string.strncpy
+    libc.src.string.strndup
     libc.src.string.strnlen
+    libc.src.string.strpbrk
+    libc.src.string.strrchr
+    libc.src.string.strsep
     libc.src.string.strspn
+    libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
+    libc.src.string.strxfrm
 
     # stdlib.h entrypoints
     libc.src.stdlib.abs
diff --git a/libc/docs/gpu/support.rst b/libc/docs/gpu/support.rst
index fd27273ed562e49..beca7b587a05215 100644
--- a/libc/docs/gpu/support.rst
+++ b/libc/docs/gpu/support.rst
@@ -45,37 +45,48 @@ string.h
 Function Name  Available  RPC Required
 =============  =========  ============
 bcmp           |check|
+bcopy          |check|
 bzero          |check|
+index          |check|
 memccpy        |check|
-memchr         
+memchr         |check|
 memcmp         |check|
 memcpy         |check|
+memmem         |check|
 memmove        |check|
 mempcpy        |check|
-memrchr
+memrchr        |check|
 memset         |check|
+rindex         |check|
 stpcpy         |check|
 stpncpy        |check|
+strcasecmp     |check|
+strcasestr     |check|
 strcat         |check|
-strchr         
+strchr         |check|
+strchrnul      |check|
 strcmp         |check|
+strcoll        |check|
 strcpy         |check|
 strcspn        |check|
+strdup         |check|
 strlcat        |check|
 strlcpy        |check|
 strlen         |check|
+strncasecmp    |check|
 strncat        |check|
 strncmp        |check|
 strncpy        |check|
+strndup        |check|
 strnlen        |check|
-strpbrk        
-strrchr        
+strpbrk        |check|
+strrchr        |check|
+strsep         |check|
 strspn         |check|
-strstr         
+strstr         |check|
 strtok         |check|
 strtok_r       |check|
-strdup
-strndup
+strxfrm        |check|
 =============  =========  ============
 
 stdlib.h

[jhuber6]

Invalidated by a merged PR.