[llvm] Enable logf128 constant folding for hosts with 128bit long double (PR #104929)

[Matthew Devereau via llvm-commits]

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@@ -271,6 +260,13 @@ if(C_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unguarded-availability-new")
 endif()
 
+check_cxx_symbol_exists(logf128 cmath HAS_LOGF128)
+check_symbol_exists(__powerpc64le__ "" __PPC64LE)
+if(HAS_LOGF128 AND NOT __PPC64LE)
+    set(LLVM_HAS_LOGF128 On)
+    add_compile_definitions(HAS_LOGF128)
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MDevereau wrote:

This was meant to always be enabled if available, and thus cmake would do a test and define the cmake variable LLVM_HAS_LOGF128 if the test passed. This patch has been reverted now, but what is likely to happen is this behaviour will be enabled by default, but be eligible to opt-out via a cmake definition which will have an entry in `llvm/Config/config.h.cmake`

https://github.com/llvm/llvm-project/pull/104929