RTL alias analysis

[Steven Bosscher <steven dot bosscher at gmail dot com>]

Hi rth,

The stack space sharing you added to cfgexpand.c breaks RTL alias
analysis.

For example, the attached test case breaks for pentiumpro at -O2. 
The problem apparently is that the second store to c is moved up
before before the load.
This looks like a serious problem to me...

Many thanks to Honza for crafting this test case.

Gr.
Steven



extern void abort (void) __attribute__((noreturn));

union setconflict
{
  short a[20];
  int b[10];
};

int
main ()
{
  int sum = 0;
  {
    union setconflict a;
    short *c;
    c = a.a;
    asm ("": "=r" (c):"0" (c));
    *c = 0;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }
  {
    union setconflict a;
    int *c;
    c = a.b;
    asm ("": "=r" (c):"0" (c));
    *c = 1;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }

  printf ("%d\n",sum);
  if (sum != 1)
    abort();
  return 0;
}



	.file	"t.c"
# GNU C version 4.2.0 20060101 (experimental) (x86_64-unknown-linux-gnu)
#	compiled by GNU C version 4.0.2 20050901 (prerelease) (SUSE Linux).
# GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
# options passed:  -iprefix -isystem -m32 -march=pentiumpro -auxbase -O2
# -fdump-tree-vars -fomit-frame-pointer -fverbose-asm
# options enabled:  -falign-loops -fargument-alias -fbranch-count-reg
# -fcaller-saves -fcommon -fcprop-registers -fcrossjumping
# -fcse-follow-jumps -fcse-skip-blocks -fdefer-pop
# -fdelete-null-pointer-checks -fearly-inlining
# -feliminate-unused-debug-types -fexpensive-optimizations -ffunction-cse
# -fgcse -fgcse-lm -fguess-branch-probability -fident -fif-conversion
# -fif-conversion2 -finline-functions-called-once -fipa-pure-const
# -fipa-reference -fipa-type-escape -fivopts -fkeep-static-consts
# -fleading-underscore -floop-optimize -floop-optimize2 -fmath-errno
# -fmerge-constants -fomit-frame-pointer -foptimize-register-move
# -foptimize-sibling-calls -fpcc-struct-return -fpeephole -fpeephole2
# -fregmove -freorder-blocks -freorder-functions -frerun-cse-after-loop
# -frerun-loop-opt -fsched-interblock -fsched-spec
# -fsched-stalled-insns-dep -fschedule-insns2 -fshow-column
# -fsplit-ivs-in-unroller -fstrength-reduce -fstrict-aliasing
# -fthread-jumps -ftrapping-math -ftree-ccp -ftree-ch -ftree-copy-prop
# -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre
# -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize -ftree-lrs
# -ftree-pre -ftree-salias -ftree-sink -ftree-sra -ftree-store-ccp
# -ftree-store-copy-prop -ftree-ter -ftree-vect-loop-version -ftree-vrp
# -funit-at-a-time -fverbose-asm -fzero-initialized-in-bss -m32 -m80387
# -m96bit-long-double -maccumulate-outgoing-args -malign-stringops
# -mfancy-math-387 -mfp-ret-in-387 -mieee-fp -mno-red-zone -mpush-args
# -mtls-direct-seg-refs

# Compiler executable checksum: 6d90f1c30ff8027bc6976ab2dbfe2320

	.section	.rodata.str1.1,"aMS",@progbits,1
.LC0:
	.string	"%d\n"
	.text
	.p2align 4,,15
.globl main
	.type	main, @function
main:
	leal	4(%esp), %ecx	#,
	andl	$-16, %esp	#,
	pushl	-4(%ecx)	#
	subl	$76, %esp	#,
	leal	28(%esp), %eax	#, tmp64
	movl	%ecx, 68(%esp)	#,
	movl	%eax, %edx	# tmp64, c
	movl	%ebx, 72(%esp)	#,
	movw	$0, (%edx)	#,* c
	movl	$1, (%eax)	#,* c
	movswl	(%edx),%ebx	#* c, sum
	movl	(%eax), %edx	#* c,
	movl	$.LC0, (%esp)	#,
	addl	%edx, %ebx	#, sum
	movl	%ebx, 4(%esp)	# sum,
	call	printf	#
	decl	%ebx	# sum
	jne	.L6	#,
	movl	68(%esp), %ecx	#,
	xorl	%eax, %eax	# <result>
	movl	72(%esp), %ebx	#,
	addl	$76, %esp	#,
	leal	-4(%ecx), %esp	#,
	ret
.L6:
	call	abort	#
	.size	main, .-main
	.ident	"GCC: (GNU) 4.2.0 20060101 (experimental)"
	.section	.note.GNU-stack,"",@progbits