Hi!
fold_binary_loc for (X >> C1) & C2, if X is zero extended narrower
value, decreases prec, but if the shift count is bigger
than the narrower prec, we then attempt to zerobits <<= negative_value.
In that case the result is necessarily zero though, all possibly non-zero
bits are shifted away, so this patch fixes this case by making sure zerobits
is all ones in that case, which results in (X, 0) folding a few lines below.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/4.9/4.8?
2014-05-15 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/61158
* fold-const.c (fold_binary_loc): If X is zero-extended and
shiftc >= prec, make sure zerobits is all ones instead of
invoking undefined behavior.
* gcc.dg/pr61158.c: New test.
--- gcc/fold-const.c.jj 2014-05-14 09:46:07.000000000 +0200
+++ gcc/fold-const.c 2014-05-14 11:35:19.593730226 +0200
@@ -11972,11 +11972,17 @@ fold_binary_loc (location_t loc,
/* See if we can shorten the right shift. */
if (shiftc < prec)
shift_type = inner_type;
+ /* Otherwise X >> C1 is all zeros, so we'll optimize
+ it into (X, 0) later on by making sure zerobits
+ is all ones. */
}
}
zerobits = ~(unsigned HOST_WIDE_INT) 0;
- zerobits >>= HOST_BITS_PER_WIDE_INT - shiftc;
- zerobits <<= prec - shiftc;
+ if (shiftc < prec)
+ {
+ zerobits >>= HOST_BITS_PER_WIDE_INT - shiftc;
+ zerobits <<= prec - shiftc;
+ }
/* For arithmetic shift if sign bit could be set, zerobits
can contain actually sign bits, so no transformation is
possible, unless MASK masks them all away. In that
@@ -11994,7 +12000,7 @@ fold_binary_loc (location_t loc,
/* ((X << 16) & 0xff00) is (X, 0). */
if ((mask & zerobits) == mask)
return omit_one_operand_loc (loc, type,
- build_int_cst (type, 0), arg0);
+ build_int_cst (type, 0), arg0);
newmask = mask | zerobits;
if (newmask != mask && (newmask & (newmask + 1)) == 0)
--- gcc/testsuite/gcc.dg/pr61158.c.jj 2014-05-14 12:18:06.066817887 +0200
+++ gcc/testsuite/gcc.dg/pr61158.c 2014-05-14 12:18:48.046598622 +0200
@@ -0,0 +1,12 @@
+/* PR tree-optimization/61158 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-original" } */
+
+unsigned long long
+foo (unsigned int x)
+{
+ return ((unsigned long long) x & 0x00ff000000000000ULL) >> 40;
+}
+
+/* { dg-final { scan-tree-dump "return 0;" "original" { target { ilp32 || lp64
} } } } */
+/* { dg-final { cleanup-tree-dump "original" } } */
Jakub
