[libc] Disable -ffreestanding and -fno-builtin on the GPU build

[jhuber6]

Summary:
This patch removed the -ffreestanding and -fno-builtin flags from
the publically installed version of the GPU library. The presense of
-fno-builtin caused issues that prevented all inlining in the GPU C
library, see https://discourse.llvm.org/t/rfc-libc-ffreestanding-fno-builtin/75883.
Previously, I attempted to fix this by loosening the restriction that
"no-builtins" functions cannot be inlined into functions without that
attribute. However, that opened up a lot of extra issues that stalled
that approach.

This patch instead removes that and instead passes -fno-builtin-<xyz>
for the few calls that are known to be problematic. I believe this works
in general as the GPU backends do not emit any libcalls and the
implementations of most of these simply reduce to builtins right now.
This is a very useful patch as we can now actually inline calls.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Joseph Huber (jhuber6)

Changes

Summary:
This patch removed the -ffreestanding and -fno-builtin flags from
the publically installed version of the GPU library. The presense of
-fno-builtin caused issues that prevented all inlining in the GPU C
library, see https://discourse.llvm.org/t/rfc-libc-ffreestanding-fno-builtin/75883.
Previously, I attempted to fix this by loosening the restriction that
"no-builtins" functions cannot be inlined into functions without that
attribute. However, that opened up a lot of extra issues that stalled
that approach.

This patch instead removes that and instead passes -fno-builtin-<xyz>
for the few calls that are known to be problematic. I believe this works
in general as the GPU backends do not emit any libcalls and the
implementations of most of these simply reduce to builtins right now.
This is a very useful patch as we can now actually inline calls.

---

Full diff: https://github.com/llvm/llvm-project/pull/97636.diff

1 Files Affected:

• (modified) libc/cmake/modules/LLVMLibCCompileOptionRules.cmake (+23-3)

diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 28379213029a3..026f54f337ac1 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -43,9 +43,11 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fpie")
 
     if(LLVM_LIBC_FULL_BUILD)
+      # Only add -ffreestanding flag in non-GPU full build mode.
+      if(LIBC_TARGET_OS_IS_GPU)
+        list(APPEND compile_options "-ffreestanding")
+      endif()
       list(APPEND compile_options "-DLIBC_FULL_BUILD")
-      # Only add -ffreestanding flag in full build mode.
-      list(APPEND compile_options "-ffreestanding")
       # Manually disable standard include paths to prevent system headers from
       # being included.
       if(LIBC_CC_SUPPORTS_NOSTDLIBINC)
@@ -60,7 +62,25 @@ function(_get_common_compile_options output_var flags)
       list(APPEND compile_options "-ffixed-point")
     endif()
 
-    list(APPEND compile_options "-fno-builtin")
+    # Builtin recognition causes issues when trying to implement the builtin
+    # functions themselves. The GPU backends do not use libcalls so we disable
+    # the known problematic ones. This allows inlining during LTO linking.
+    if(LIBC_TARGET_OS_IS_GPU)
+      list(APPEND compile_options "-fno-builtin-bcmp")
+      list(APPEND compile_options "-fno-builtin-strlen")
+      list(APPEND compile_options "-fno-builtin-memmem")
+      list(APPEND compile_options "-fno-builtin-bzero.h")
+      list(APPEND compile_options "-fno-builtin-memcmp.h")
+      list(APPEND compile_options "-fno-builtin-memcpy.h")
+      list(APPEND compile_options "-fno-builtin-memmem.h")
+      list(APPEND compile_options "-fno-builtin-memmove.h")
+      list(APPEND compile_options "-fno-builtin-memset.h")
+      list(APPEND compile_options "-fno-builtin-strcmp.h")
+      list(APPEND compile_options "-fno-builtin-strstr.h")
+    else()
+      list(APPEND compile_options "-fno-builtin")
+    endif()
+
     list(APPEND compile_options "-fno-exceptions")
     list(APPEND compile_options "-fno-lax-vector-conversions")
     list(APPEND compile_options "-fno-unwind-tables")

[michaelrj-google]

LGTM from the libc side