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optimization/3756: gcc wishlist: arithmetic right shift for ternary operator
- To: gcc-gnats at gcc dot gnu dot org
- Subject: optimization/3756: gcc wishlist: arithmetic right shift for ternary operator
- From: Kevin Ryde <user42 at zip dot com dot au>
- Date: 21 Jul 2001 11:17:59 +1000
>Number: 3756
>Category: optimization
>Synopsis: gcc wishlist: arithmetic right shift for ternary operator
>Confidential: yes
>Severity: non-critical
>Priority: low
>Responsible: unassigned
>State: open
>Class: pessimizes-code
>Submitter-Id: net
>Arrival-Date: Fri Jul 20 18:26:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:
>Release: 3.0 (Debian) (Debian testing/unstable)
>Organization:
>Environment:
System: Linux blah 2.2.15 #1 Tue Apr 25 17:13:48 EST 2000 i586 unknown
Architecture: i586
<machine, os, target, libraries (multiple lines)>
host: i386-pc-linux-gnu
build: i386-pc-linux-gnu
target: i386-pc-linux-gnu
configured with: ../src/configure -v --enable-languages=c,c++,java,f77,proto,objc --prefix=/usr --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as --with-gnu-ld --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --disable-checking --enable-threads=posix --enable-java-gc=boehm --with-cpp-install-dir=bin --enable-objc-gc i386-linux
>Description:
It seems that, on i386 at least, gcc misses the opportunity to use an
arithmetic right shift when compiling a ternary expression like "n >=
0 ? a : b", for constant a and b.
>How-To-Repeat:
An example I first struck was
int
pn (int n)
{
return (n >= 0 ? 1 : -1);
}
compiled with
gcc-3.0 -O9 -fomit-frame-pointer -S pn.c
giving
.file "pn.c"
.text
.align 4
.globl pn
.type pn,@function
pn:
movl 4(%esp), %eax
notl %eax
shrl $31, %eax
leal -1(%eax,%eax), %eax
ret
.Lfe1:
.size pn,.Lfe1-pn
.ident "GCC: (GNU) 3.0 (Debian)"
I think instead of notl/shrl/leal it could do
sarl $31, %eax
orl $1, %eax
If I'm not mistaken "or" and "and" get used for that sort of thing
already, it's just the sarl that's missed.
A more general example
int
sel (int n)
{
return (n >= 0 ? 60 : 100);
}
gives a slightly different form
shrl $31, %eax
decl %eax
andl $-40, %eax
addl $100, %eax
Again I think again the shrl+decl could be a sarl (and the constants
adjusted accordingly).
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: