diff options
Diffstat (limited to 'gcc/tree-ssa-loop-niter.c')
| -rw-r--r-- | gcc/tree-ssa-loop-niter.c | 21 |
1 files changed, 13 insertions, 8 deletions
diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c index 8399faf..ce4b3cf 100644 --- a/gcc/tree-ssa-loop-niter.c +++ b/gcc/tree-ssa-loop-niter.c @@ -1891,19 +1891,24 @@ scev_probably_wraps_p (tree type, tree base, tree step, /* If AT_STMT represents a cast operation, we may not be able to take advantage of the undefinedness of signed type evolutions. + + implement-c.texi states: "For conversion to a type of width + N, the value is reduced modulo 2^N to be within range of the + type;" + See PR 21959 for a test case. Essentially, given a cast operation - unsigned char i; - signed char i.0; + unsigned char uc; + signed char sc; ... - i.0_6 = (signed char) i_2; - if (i.0_6 < 0) + sc = (signed char) uc; + if (sc < 0) ... - where i_2 and i.0_6 have the scev {0, +, 1}, we would consider - i_2 to wrap around, but not i.0_6, because it is of a signed - type. This causes VRP to erroneously fold the predicate above - because it thinks that i.0_6 cannot be negative. */ + where uc and sc have the scev {0, +, 1}, we would consider uc to + wrap around, but not sc, because it is of a signed type. This + causes VRP to erroneously fold the predicate above because it + thinks that sc cannot be negative. */ if (at_stmt && TREE_CODE (at_stmt) == MODIFY_EXPR) { tree rhs = TREE_OPERAND (at_stmt, 1); |