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problem with zero_extract during gcse


Hi,

I have a problem with 4.1 on m68k-linux, which miscompiles the following 
test case during the gcse pass:

struct b {
	unsigned a : 1;
	unsigned b : 1;
	unsigned c : 1;
	unsigned d : 1;
};

unsigned int x = 1;

void f(int y, struct b *p)
{       
	switch (y) {
	case 1: 
		p->a = 0;
		p->b = 0;
		break;
	case 3: 
		p->a = 0;
		p->b = 1;
		break;
	default:
		return;
	}
	p->c = x;
	p->d = 1;
}

The assignment to p->c is done via zero_extract:

(insn 46 45 48 5 (set (zero_extract:SI (mem/s/j:QI (reg/v/f:SI 31 [ p ]) [0 S1 A8])
            (const_int 1 [0x1])
            (const_int 2 [0x2]))
        (reg:SI 40 [ x ])) 278 {*m68k.md:4815} (nil)
    (nil))

The other assignments are done with (and) and (ior). When propagating the 
mem expression, gcse misses this assignment and the assignment to p->d 
overwrites it, because the earlier mem expression was propagated past it 
in a register.

Currently I'm testing the patch below, which simply invalidates the 
load/store. Now I need some help from someone, who is more familiar with 
this code, whether this is the correct approach.
It would be nice if above could be changed into (zero_extract:SI (reg)), 
but I guess that would be a little too complex.

bye, Roman

Index: gcc/gcse.c
===================================================================
--- gcc/gcse.c	(revision 116085)
+++ gcc/gcse.c	(working copy)
@@ -5319,6 +5319,16 @@
 		      else
 			ptr->invalid = 1;
 		    }
+		  else if (GET_CODE (dest) == ZERO_EXTRACT)
+		    {
+		      dest = XEXP (dest, 0);
+		      if (MEM_P (dest) && simple_mem (dest))
+			{
+			  ptr = ldst_entry (dest);
+			  ptr->invalid = 1;
+			}
+
+		    }
 		}
 	      else
 		invalidate_any_buried_refs (PATTERN (insn));


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