http://llvm.org/bugs/show_bug.cgi?id=11289
Bug #: 11289
Summary: Inefficient x86 vector code generation for compare
less than 0
Product: libraries
Version: trunk
Platform: Macintosh
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
Code generated for a vector comparison to zero is inefficient.
For example the C code below creates mask where each result vector element is
equal to -1 if the input vector element is negative, and 0 otherwise.
#include <emmintrin.h>
void test(__m128i* p)
{
*p = _mm_cmplt_epi8(*p, _mm_setzero_si128());
}
Using http://llvm.org/demo/index.cgi the generated LLVM assembly is
; ModuleID = '/tmp/webcompile/_11830_0.bc'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"
define void @test(<2 x i64>* nocapture %p) nounwind {
%1 = load <2 x i64>* %p, align 16, !tbaa !0
%2 = bitcast <2 x i64> %1 to <16 x i8>
%.lobit.i.i = ashr <16 x i8> %2, <i8 7, i8 7, i8 7, i8 7, i8 7, i8 7, i8 7,
i8 7, i8 7, i8 7, i8 7, i8 7, i8 7, i8 7, i8 7, i8 7>
%3 = bitcast <16 x i8> %.lobit.i.i to <2 x i64>
store <2 x i64> %3, <2 x i64>* %p, align 16, !tbaa !0
ret void
}
!0 = metadata !{metadata !"omnipotent char", metadata !1}
!1 = metadata !{metadata !"Simple C/C++ TBAA", null}
where the comparison < 0 is replaced by >> 7
Alternatively the vector comparison could also be written as
define <16 x i8> @test(<16 x i8> %a) {
%b = icmp sgt <16 x i8> zeroinitializer, %a
%c = sext <16 x i1> %b to <16 x i8>
ret <16 x i8> %c
}
In both cases the generated code is convoluted where individual vector elements
are extracted using pextrb instructions, shifted using sarb instructions, and
reinserted using pinsrb instructions.
This was tested using r143475 from trunk.
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