optimization/7476: constant variable tested in loop

[Daniel Berlin]

Fixed by the local cprop stuff Jan added to the mainline.
        .file   "testconst.c"
        .text
        .align 2
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
        pushl   %ebp
        movl    $9, %eax
        movl    %esp, %ebp
        .p2align 4,,15
.L7:
        decl    %eax
        jns     .L7
        popl    %ebp
        ret
        .size   foo, .-foo
        .ident  "GCC: (GNU) 3.3 20020729 (experimental)

On Sat, 3 Aug 2002, Kevin Ryde wrote:

> 
> >Number:         7476
> >Category:       optimization
> >Synopsis:       constant variable tested in loop
> >Confidential:   no
> >Severity:       non-critical
> >Priority:       low
> >Responsible:    unassigned
> >State:          open
> >Class:          pessimizes-code
> >Submitter-Id:   net
> >Arrival-Date:   Fri Aug 02 16:06:01 PDT 2002
> >Closed-Date:
> >Last-Modified:
> >Originator:     
> >Release:        3.1.1 20020606 (Debian prerelease) (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: /mnt/data/gcc-3.1/gcc-3.1-3.1.1ds1/src/configure -v --enable-languages=c,c++,java,f77,proto,objc,ada --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-dir=/usr/include/g++-v3-3.1 --enable-shared --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --enable-clocale=gnu --enable-threads=posix --enable-java-gc=boehm --enable-objc-gc i386-linux
> 
> >Description:
> A constant variable tested within a loop doesn't seem to be
> recognised as constant.
> 
> >How-To-Repeat:
> The file foo.c below compiled with
> 
> 	gcc-3.1 -O9 -S foo.c
> 
> produces the foo.s below.  Notice that it still contains a call to
> "puts", despite the "cond" variable in the source always being false.
> 
> Outside a loop such a construct seems to be recognised as constant and
> the dependent code goes dead, but within a loop that doesn't seem to
> happen.
> 
> I'm not sure if it's too much to expect something like this in a loop
> to go dead.  I guess in the presence of gotos or loops it's not always
> too easy to be certain something really is constant.  A variable
> written only once with a constant value like this might be tractable
> though.
> 
> For what it's worth this arose within gmp where the "cond" condition
> tested was sometimes a compile-time constant, or sometimes a runtime
> test, depending on the configuration.  It had seemed like a good idea
> just to put the result in a variable in either case, and let the
> optimizer drop unused code in the constant case.
> 
> 
> 
> --=-=-=
> Content-Type: text/x-csrc
> Content-Disposition: attachment; filename=foo.c
> 
> void
> foo (void)
> {
>   int  cond = 0;
>   int  i;
> 
>   for (i = 0; i < 10; i++)
>     if (cond)
>       puts ("hello");
> }
> 
> --=-=-=
> Content-Disposition: attachment; filename=foo.s
> 
> 	.file	"foo.c"
> 	.section	.rodata.str1.1,"aMS",@progbits,1
> .LC0:
> 	.string	"hello"
> 	.text
> 	.align 2
> 	.p2align 2,,3
> .globl foo
> 	.type	foo,@function
> foo:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	pushl	%esi
> 	pushl	%ebx
> 	xorl	%esi, %esi
> 	movl	$9, %ebx
> 	.p2align 2,,3
> .L7:
> 	testl	%esi, %esi
> 	jne	.L10
> .L4:
> 	decl	%ebx
> 	jns	.L7
> 	leal	-8(%ebp), %esp
> 	popl	%ebx
> 	popl	%esi
> 	leave
> 	ret
> 	.p2align 2,,3
> .L10:
> 	subl	$12, %esp
> 	pushl	$.LC0
> 	call	puts
> 	addl	$16, %esp
> 	jmp	.L4
> .Lfe1:
> 	.size	foo,.Lfe1-foo
> 	.ident	"GCC: (GNU) 3.1.1 20020703 (Debian prerelease)"
> 
> --=-=-=--
> >Fix:
> >Release-Note:
> >Audit-Trail:
> >Unformatted:
>  --=-=-=
>  
> 
>