https://llvm.org/bugs/show_bug.cgi?id=24562
Bug ID: 24562
Summary: pshufb intrinsic doesn't understand that indices with
high-bit set give zeros
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: dbau...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 14767
--> https://llvm.org/bugs/attachment.cgi?id=14767&action=edit
C file to compare clang/LLVM and GCC
The `pshufb` SSSE3 instruction takes two vectors of i8's, x and y, and returns
<x[y[0] % 16], x[y[1] % 16], ..., x[y[15] % 16]>, except for indices where the
high bit is set, which return zero.
LLVM seems to ignore the high-bit, and will lower calls to
`llvm.x86.ssse3.pshuf.b.128` as if they unconditionally indexed with the mask.
The attached pshufb.c file prints "1 1 ... 1" with clang, and "0 0 ... 0" with
gcc.
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