On 5/8/19 4:31 PM, Richard Biener wrote: > On Tue, May 7, 2019 at 4:34 AM JunMa <ju...@linux.alibaba.com> wrote: >> >> 在 2019/5/6 下午7:58, JunMa 写道: >>> 在 2019/5/6 下午6:02, Richard Biener 写道: >>>> On Thu, Mar 21, 2019 at 5:57 AM JunMa <ju...@linux.alibaba.com> wrote: >>>>> Hi >>>>> For now, gcc can not fold code like: >>>>> >>>>> const char a[5] = "123" >>>>> __builtin_memchr (a, '7', sizeof a) >>>>> >>>>> It tries to avoid folding out of string length although length of a >>>>> is 5. >>>>> This is a bit conservative, it's safe to folding memchr/bcmp/memcmp >>>>> builtins when constant string stores in array with some trailing nuls. >>>>> >>>>> This patch folds these cases by exposing additional length of >>>>> trailing nuls in c_getstr(). >>>>> Bootstrapped/regtested on x86_64-linux, ok for trunk? >>>> It's probably better if gimple_fold_builtin_memchr uses string_constant >>>> directly instead? >>> We can use string_constant in gimple_fold_builtin_memchr, however it is a >>> bit complex to use it in memchr/memcmp constant folding. >>>> You are changing memcmp constant folding but only have a testcase >>>> for memchr. >>> I'll add later. >>>> If we decide to amend c_getstr I would rather make it return the >>>> total length instead of the number of trailing zeros. >>> I think return the total length is safe in other place as well. >>> I used the argument in patch since I do not want any impact on >>> other part at all. >>> >> >> Although it is safe to use total length, I found that function >> inline_expand_builtin_string_cmp() which used c_getstr() may emit >> redundant rtls for trailing null chars when total length is returned. >> >> Also it is trivial to handle constant string with trailing null chars. >> >> So this updated patch follow richard's suggestion: using string_constant >> directly. >> >> Bootstrapped/regtested on x86_64-linux, ok for trunk? > > Doesn't this fold to NULL for the case where seaching for '0' and it > doesn't occur in the string constant but only the zero-padding? > So you'd need to conditionalize on c being not zero (or handle > that case specially by actually finding the first zero in the padding)? > I think there was work to have all string constants zero terminated > but I'm not sure if we can rely on that here. Bernd? >
It turned out that there is a number of languages that don't have zero-terminated strings by default, which would have complicated the patch just too much for too little benefit. In the end, it was more important to guarantee that mem_size >= string_length holds. In C it is just a convention that string constants have usually a zero-termination, but even with C there are ways how strings constants can be not zero-terminated. There can in theory be optimizations that introduce not zero-terminated strings, like tree-ssa-forwprop.c, where a not zero-terminated string constant is folded in simplify_builtin_call. In such a case, c_getstr might in theory return a string without zero-termination, but I think it will be rather difficult to find a C test case for that. Well, if I had a test case for that I would probably fix it in c_getstr to consider the implicit padding as equivalent to an explicit zero-termination. Bernd. > Richard. > >> Regards >> JunMa >> >> gcc/ChangeLog >> >> 2019-05-07 Jun Ma <ju...@linux.alibaba.com> >> >> PR Tree-optimization/89772 >> * gimple-fold.c (gimple_fold_builtin_memchr): consider trailing nuls in >> out-of-bound accesses checking. >> >> gcc/testsuite/ChangeLog >> >> 2019-05-07 Jun Ma <ju...@linux.alibaba.com> >> >> PR Tree-optimization/89772 >> * gcc.dg/builtin-memchr-4.c: New test. >>> Thanks >>> JunMa >>>> Richard. >>>> >>>>> Regards >>>>> JunMa >>>>> >>>>> >>>>> gcc/ChangeLog >>>>> >>>>> 2019-03-21 Jun Ma <ju...@linux.alibaba.com> >>>>> >>>>> PR Tree-optimization/89772 >>>>> * fold-const.c (c_getstr): Add new parameter to get length of >>>>> additional >>>>> trailing nuls after constant string. >>>>> * gimple-fold.c (gimple_fold_builtin_memchr): consider >>>>> trailing nuls in >>>>> out-of-bound accesses checking. >>>>> * fold-const-call.c (fold_const_call): Likewise. >>>>> >>>>> >>>>> gcc/testsuite/ChangeLog >>>>> >>>>> 2019-03-21 Jun Ma <ju...@linux.alibaba.com> >>>>> >>>>> PR Tree-optimization/89772 >>>>> * gcc.dg/builtin-memchr-4.c: New test. >>> >>
