c/2480: aliasing problem with global structures

[Dan Nicolaescu <dann@godzilla.ics.uci.edu>]

>Number:         2480
>Category:       c
>Synopsis:       aliasing problem with global structures
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Tue Apr 03 16:55:59 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Dan Nicolaescu <dann@godzilla.ics.uci.edu>
>Release:        gcc version 3.1 20010402 (experimental)
>Organization:
>Environment:
sun-sparc-solaris2.7
>Description:
The following code: 

struct example
{
  char a;
  int  b;
  char c;
} *ex1;

void
bar (void)
{
  ex1->a = 1;
  ex1->b = 2;
  ex1->c = 3;
}

void
foo (struct example *ex2)
{
  ex2->a = 1;
  ex2->b = 2;
  ex2->c = 3;
}

When compiled with -O2 -fstrict-aliasing -S on sun-sparc-solaris2.7 
with a GCC mainline snapshot from 2000-04-02 
(but the same problem occurs on the GCC-3.0 branch and 2.95.2)


bar:
	!#PROLOGUE# 0
	!#PROLOGUE# 1
	sethi	%hi(ex1), %o2
	ld	[%o2+%lo(ex1)], %o1
	mov	1, %o0
	stb	%o0, [%o1]
	ld	[%o2+%lo(ex1)], %o3
	^^^^^
	after the store ex1 is reloaded. 
	true_dependence returns true for these last 2 instructions, it
	seems that the ex1->a is put in the alias set 0. That is a
	mistake, but I couldn't find where that is done....
	
	It looks like GCC treats ex1->a as a char*, but that is
	incorrect, a store to ex1->a cannot alias ex1

	mov	3, %o0
	mov	2, %o1
	st	%o0, [%o3+8]
	^^^ 
	After this store ex1 is not reloaded, but in this case the
	struct member is an "int"
	
	retl
	st	%o1, [%o3+4]


foo:
	!#PROLOGUE# 0
	!#PROLOGUE# 1
	mov	3, %o1
	st	%o1, [%o0+8]
	mov	1, %o2
	mov	2, %o1
	stb	%o2, [%o0]
	retl
	st	%o1, [%o0+4]

nothing like that happens here, when the pointer to the structure is
passed as a parameter. 


This is an important pessimization. 
GCC itself contains a lot of global pointers to structures...


>How-To-Repeat:
Compile the code in the description with -O2 -S -fstrict-aliasing
and look at the resulting assembly
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: