llvm-project/llvm/lib/Target/X86

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// Random ideas for the X86 backend.
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Add a MUL2U and MUL2S nodes to represent a multiply that returns both the
Hi and Lo parts (combination of MUL and MULH[SU] into one node).  Add this to
X86, & make the dag combiner produce it when needed.  This will eliminate one
imul from the code generated for:

long long test(long long X, long long Y) { return X*Y; }

by using the EAX result from the mul.  We should add a similar node for
DIVREM.

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This should be one DIV/IDIV instruction, not a libcall:

unsigned test(unsigned long long X, unsigned Y) {
        return X/Y;
}

This can be done trivially with a custom legalizer.  What about overflow 
though?  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224

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Need to add support for rotate instructions.

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Some targets (e.g. athlons) prefer freep to fstp ST(0):
http://gcc.gnu.org/ml/gcc-patches/2004-04/msg00659.html

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This should use faddi on chips where it is profitable:
double foo(double P, int *I) { return P+*I; }

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The FP stackifier needs to be global.  Also, it should handle simple permutates
to reduce number of shuffle instructions, e.g. turning:

fld P	->		fld Q
fld Q			fld P
fxch

or:

fxch	->		fucomi
fucomi			jl X
jg X

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Improvements to the multiply -> shift/add algorithm:
http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html

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Improve code like this (occurs fairly frequently, e.g. in LLVM):
long long foo(int x) { return 1LL << x; }

http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html

Another useful one would be  ~0ULL >> X and ~0ULL << X.

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Should support emission of the bswap instruction, probably by adding a new
DAG node for byte swapping.  Also useful on PPC which has byte-swapping loads.