[LLVMdev] Clang and i128

[Mario Schwalbe]

Hi all,

I currently use LLVM 3.0 clang to compile my source code to bitcode
(on a X86-64 machine) before it is later processed by a pass, like this:

$ clang -m32 -O3 -S foo.c -emit-llvm -o foo.ll

However, for some reason the the resulting module contains 128-bit
instructions, e.g.:

%6 = load i8* %arrayidx.1.i, align 1, !tbaa !0
%7 = zext i8 %6 to i128
%8 = shl nuw nsw i128 %7, 8

which the pass can't handle (and never will).

So my question is: Why does it do so? The code doesn't use integer types
larger than 32-bit. Is there an option to prevent clang from using
those types? If no, which pass might be responsible for this kind of
optimization?

Thanks in advance,
Mario
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[Rotem, Nadav]

Mario,

The ScalarReplAggregates pass attempts to converts structs into scalars to allow many optimizations. Try running this pass with a different threshold or try placing a breakpoint on ConvertScalar_ExtractValue and check if you can manually disable some of the transformations in SRA.

Nadav

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