This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug target/62191] New: extra shift generated for vector integer division by constant 2
- From: "spatel at rotateright dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Tue, 19 Aug 2014 18:24:06 +0000
- Subject: [Bug target/62191] New: extra shift generated for vector integer division by constant 2
- Auto-submitted: auto-generated
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62191
Bug ID: 62191
Summary: extra shift generated for vector integer division by
constant 2
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: spatel at rotateright dot com
Using gcc 4.9:
$ cat sdiv.c
typedef int vecint __attribute__((vector_size(16)));
vecint f(vecint x) {
return x/2;
}
$ gcc -O2 sdiv.c -S -o -
...
movdqa %xmm0, %xmm1
psrad $31, %xmm1 <--- splat the sign bit
psrld $31, %xmm1 <--- then shift sign bit down to LSB
paddd %xmm1, %xmm0 <--- add sign bit to quotient
psrad $1, %xmm0 <--- div via alg shift right
ret
--------------------------------------------------------------
I don't think the first shift right algebraic is necessary. We splat the sign
bit and then shift that right logically, so the upper bits are all zero'd
anyway.
This is a special case for signed integer division by 2. You need that first
'psrad' for any other power of 2 because the subsequent logical shift would not
also be a shift of 31.