[llvm] [TLI] Add basic support for fdim libcall (PR #108702)

[Matt Arsenault via llvm-commits]

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@@ -337,6 +338,7 @@ static void initializeLibCalls(TargetLibraryInfoImpl &TLI, const Triple &T,
     TLI.setUnavailable(LibFunc_powl);
     TLI.setUnavailable(LibFunc_remainderl);
     TLI.setUnavailable(LibFunc_remquol);
+    TLI.setUnavailable(LibFunc_fdiml);
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arsenm wrote:

There seem to be 3 ways it's done. The most direct way has an example in TargetLibraryInfoTest.cpp in TLITestAarch64. 

There's also an underutilized looking direct test in llvm/test/tools/llvm-tli-checker, which currently only covers ps4.

The indirect way most TLI tests do is to add a negative instcombine test for some transform, e.g. test/Transforms/InstCombine/memcmp-1.ll

https://github.com/llvm/llvm-project/pull/108702