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re PR c/7284 (incorrectly simplifies leftshift followed by signed power-of-2 division)

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PR c/7284
	* fold-const.c (extract_muldiv_1): Do not treat signed left shift
	as multiplication.

testsuite:
	* gcc.c-torture/execute/pr7284-1.c: New test.

From-SVN: r85059
This commit is contained in:
Joseph Myers 2004-07-22 21:33:34 +01:00 • committed by Joseph Myers
parent d592f1c398
commit 9e629a806d
4 changed files with 40 additions and 1 deletions
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6
gcc/ChangeLog
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@ -1,3 +1,9 @@
2004-07-22 Joseph S. Myers <jsm@polyomino.org.uk>
PR c/7284
* fold-const.c (extract_muldiv_1): Do not treat signed left shift
as multiplication.
2004-07-22 Joseph S. Myers <jsm@polyomino.org.uk>
* doc/implement-c.texi: New file.

6
gcc/fold-const.c
View file

@ -5139,8 +5139,12 @@ extract_muldiv_1 (tree t, tree c, enum tree_code code, tree wide_type)
case LSHIFT_EXPR: case RSHIFT_EXPR:
/* If the second operand is constant, this is a multiplication
or floor division, by a power of two, so we can treat it that
way unless the multiplier or divisor overflows. */
way unless the multiplier or divisor overflows. Signed
left-shift overflow is implementation-defined rather than
undefined in C90, so do not convert signed left shift into
multiplication. */
if (TREE_CODE (op1) == INTEGER_CST
&& (tcode == RSHIFT_EXPR || TYPE_UNSIGNED (TREE_TYPE (op0)))
/* const_binop may not detect overflow correctly,
so check for it explicitly here. */
&& TYPE_PRECISION (TREE_TYPE (size_one_node)) > TREE_INT_CST_LOW (op1)

5
gcc/testsuite/ChangeLog
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@ -1,3 +1,8 @@
2004-07-22 Joseph S. Myers <jsm@polyomino.org.uk>
PR c/7284
* gcc.c-torture/execute/pr7284-1.c: New test.
2004-07-22 Brian Booth <bbooth@redhat.com>
* gcc.dg/tree-ssa/20040721-1.c: New test.

24
gcc/testsuite/gcc.c-torture/execute/pr7284-1.c Normal file
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@ -0,0 +1,24 @@
/* Signed left-shift is implementation-defined in C89 (and see
DR#081), not undefined. Bug 7284 from Al Grant (AlGrant at
myrealbox.com). */
/* { dg-options "-std=c89" } */
extern void abort (void);
extern void exit (int);
int
f (int n)
{
return (n << 24) / (1 << 23);
}
volatile int x = 128;
int
main (void)
{
if (f(x) != -256)
abort ();
exit (0);
}