Hi, > Currently, tree-ssa-loop-ivopts assumes that pointers and integers can > be used interchangeably. It prefers to compute everything in unsigned > integer types rather than pointer types. > On a new target that I'm working on, this assumption is problematic; > casting a pointer to an integer and doing arithmetic on the integer > would be much too expensive to be useful. I came up with the patch > below, which makes ivopts try harder to preserve pointer types. > tree-affine is changed to keep track of base pointers, and do arithmetic > (in particular subtraction) properly if base pointers are involved. > > Bootstrapped and regression tested with all languages on i686-linux. I > get FAILs in: > > * gcc.dg/guality/pr43051-1.c > A somewhat contrived testcase; gdb now produces "optimized out" > messages which I think is acceptable? In any case I remain to be > convinced that this testcase demonstrates any real problem. > * gcc.dg/tree-ssa/reassoc-19.c scan-tree-dump-times reassoc2 " \+ " 0 > This seems to fail because we do POINTER_PLUS (ptr, NEG offset)) > rather than (char *)((int)ptr - offset). As far as I can tell this > is also not a real problem, but I don't know how to adjust the > testcase. > > Comments? Ok?
the reason unsigned integer types are prefered is that possible overflows during the computation have defined semantics. With pointer types, the intermediate steps of the computations could have undefined behavior, possibly confusing further optimizations. Is the patch with this regard? Zdenek
