[cmake] Disable a GCC optimization when building LLVM for MIPS

[sdardis]

Summary

GCC when compiling LLVM for MIPS can introduce a jump to an uninitialized value
when shrink wrapping is enabled. As shrink wrapping is enabled in GCC at all
optimization levels, it must be disabled. This bug exists for all versions of
GCC since 4.9.2.

This partially resolves PR37701 / GCC PR target/86069.

[sdardis]

Ping.

[emaste]

I think this is reasonable.

[mstojanovic]

Ping

[beanz]

Is there a specific version range of GCC that this miscompile occurs with?

[sdardis]

In D48069#1479260, @beanz wrote:

Is there a specific version range of GCC that this miscompile occurs with?

Based on activity in the listed GCC PR/target there's been no activity on fixing the bug. GCC 4.9.2 was the version I had to hand when I posted this patch.

Looking at the blame history and a read through the GCC mailing lists shows that the current 'ghost gp' instruction implementation have been present on GCC trunk since Sept, 2009. So that'd probably mean every version since 4.5 could be affected. Older ones might not have the bug, but they're not supported for building LLVM.

I believe it would be safer to simply disable the shrink wrapping optimization outright until the issue in GCC is addressed.

[beanz]

Seems reasonable to me.

cmake/modules/HandleLLVMOptions.cmake

if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't
     # know how to disable this, so just force ENABLE_PIC off for now.
     message(WARNING "-fPIC not supported with Xcode.")
   elseif( WIN32 OR CYGWIN)
     # On Windows all code is PIC. MinGW warns if -fPIC is used.
   else()
     add_flag_or_print_warning("-fPIC" FPIC)
   endif()
+  # GCC for MIPS can miscompile LLVM due to PR37701.
+  if(CMAKE_COMPILER_IS_GNUCXX AND LLVM_NATIVE_ARCH STREQUAL "Mips" AND
+         NOT Uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
+    add_flag_or_print_warning("-fno-shrink-wrap" FNO_SHRINK_WRAP)
+  endif()
 endif()
 
 if(NOT WIN32 AND NOT CYGWIN)
   # MinGW warns if -fvisibility-inlines-hidden is used.
   check_cxx_compiler_flag("-fvisibility-inlines-hidden" SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG)
   append_if(SUPPORTS_FVISIBILITY_INLINES_HIDDEN_FLAG "-fvisibility-inlines-hidden" CMAKE_CXX_FLAGS)
 endif()