[PATCH] X86 Builtins: rewrite SSE2/AVX2 shift builtin intrinsics using shift operators

[Andrea_DiBiagio at sn.scee.net]

> From: Eli Friedman <eli.friedman at gmail.com>
> On Wed, Jul 17, 2013 at 10:44 AM,  <Andrea_DiBiagio at sn.scee.net> wrote:
> > Hi all,
> >
> > I'd like to contribute a patch that rewrites (most of the) X86
> > SSE2/AVX2 packed shifts builtin intrinsics using shift operators.
> 
> No.  They don't have the same semantics in the boundary cases.  (From
> LangRef for lshr: "If op2 is (statically or dynamically) equal to or
> larger than the number of bits in op1, the result is undefined.")
> 
> -Eli

True...

I think the only reason why it was working for me is because the 
shift-count-overflow is only spotted by the optimizer in the case of 
scalar shifts.
clang warns about shift-count-overflow in the case of scalar shifts and 
the optimizer correctly propagates undef in that case. However that does 
not seem to happen in the case of vector shifts.

//////////
typedef int int4 __attribute__((ext_vector_type(4)))

int4 foo(int4 op1) {
  return op1 << (int4){300,300,300,300};
}

int bar(int op1) {
  return op1 << 300;
}
/////
At -O1
function `bar' is optimized into
ret i32 undef

However the shift instruction in `foo' is not replaced:

%shl = shl <4 x i32> %op1, <i32 300, i32 300, i32 300, i32 300>

Anyway..
thanks for the feedback and sorry for the wrong patch.

Andrea Di Biagio
SN Systems - Sony Computer Entertainment Group

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