On Fri, Aug 14, 2026 at 2:14 PM Daniel Barboza <[email protected]> wrote: > > We're not trying to fold builtin_strnlen calls into constants like we do > with builtin_strlen. This results in missed optimizations where we > have a strnlen that uses a PHI as input: > > const char a[4] = "123"; > > int g (int i) > { > return __builtin_strnlen (i ? a : "", 4); > } > > The strnlen call could be folded into a single PHI <0, 3> but instead > we're calling strnlen with the PHI result: > > # iftmp.1_2 = PHI <&aD.4472(3), ""(2)> > # VUSE <.MEM_4(D)> > # RANGE [irange] long unsigned int [0, 4] MASK 0x7 VALUE 0x0 > # USE = nonlocal escaped const-pool { D.4472 } (nonlocal) > _1 = strnlenD.1862 (iftmp.1_2, 4); > # RANGE [irange] int [0, 4] MASK 0x7 VALUE 0x0 > _5 = (intD.7) _1; > # VUSE <.MEM_4(D)> > return _5; > > After this change: > > # RANGE [irange] int [0, 0][3, 3] MASK 0x3 VALUE 0x0 > # prephitmp_7 = PHI <3(3), i_3(D)(2)> > # VUSE <.MEM_4(D)> > return prephitmp_7; > > Bootstrapped and regression tested in x86_64.
Ok. > > PR tree-optimization/86937 > > gcc/ChangeLog: > > * fold-const-call.cc (fold_const_call): fold > CFN_BUILT_IN_STRNLEN calls. > > gcc/testsuite/ChangeLog: > > * gcc.dg/warn-strnlen-no-nul.c: warnings were > added/removed/changed due to the new STRNLEN folding. > * gcc.dg/tree-ssa/pr86937.c: New test. > --- > > Changes from v1: > - add extra space in "strnlen " scan > - remove the added xfail tests in warn-strnlen-no-nul.c > - v1 link: https://gcc.gnu.org/pipermail/gcc-patches/2026-August/727358.html > > gcc/fold-const-call.cc | 11 ++++++++ > gcc/testsuite/gcc.dg/tree-ssa/pr86937.c | 15 +++++++++++ > gcc/testsuite/gcc.dg/warn-strnlen-no-nul.c | 29 +++++++++++----------- > 3 files changed, 41 insertions(+), 14 deletions(-) > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr86937.c > > diff --git a/gcc/fold-const-call.cc b/gcc/fold-const-call.cc > index b3a9d7715d6..c483b80e704 100644 > --- a/gcc/fold-const-call.cc > +++ b/gcc/fold-const-call.cc > @@ -1911,6 +1911,17 @@ fold_const_call (combined_fn fn, tree type, tree arg0, > tree arg1) > } > return NULL_TREE; > > + case CFN_BUILT_IN_STRNLEN: > + if ((p0 = c_getstr (arg0))) > + { > + unsigned HOST_WIDE_INT s1 = 0; > + if (!size_t_cst_p (arg1, &s1)) > + return NULL_TREE; > + > + return build_int_cst (type, strnlen (p0, s1)); > + } > + return NULL_TREE; > + > case CFN_FOLD_LEFT_PLUS: > return fold_const_fold_left (type, arg0, arg1, PLUS_EXPR); > > diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr86937.c > b/gcc/testsuite/gcc.dg/tree-ssa/pr86937.c > new file mode 100644 > index 00000000000..b6b2876bf64 > --- /dev/null > +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr86937.c > @@ -0,0 +1,15 @@ > +/* { dg-do compile } */ > +/* { dg-options "-O2 -fdump-tree-pre" } */ > +const char a[4] = "123"; > + > +int f (int i) > +{ > + return __builtin_strlen (i ? a : ""); > +} > + > +int g (int i) > +{ > + return __builtin_strnlen (i ? a : "", 4); > +} > + > +/* { dg-final { scan-tree-dump-times "strnlen " 0 "pre" } } */ > diff --git a/gcc/testsuite/gcc.dg/warn-strnlen-no-nul.c > b/gcc/testsuite/gcc.dg/warn-strnlen-no-nul.c > index 70f6a432b97..45a948c22b6 100644 > --- a/gcc/testsuite/gcc.dg/warn-strnlen-no-nul.c > +++ b/gcc/testsuite/gcc.dg/warn-strnlen-no-nul.c > @@ -147,14 +147,14 @@ T (v0 ? b[3] : "", bsz); > the strnlen calls are safe because the reads are bounded by > the length of the constant arguments. Most of the calls are > not diagnosed anymore as a result of the fix for PR 103215. */ > -T (v0 ? "" : b[0], bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" } */ > +T (v0 ? "" : b[0], bsz + 1); > T (v0 ? "" : b[1], bsz + 1); > T (v0 ? "" : b[2], bsz + 1); > -T (v0 ? "" : b[3], bsz + 1); > -T (v0 ? b[0] : "", bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" } */ > +T (v0 ? "" : b[3], bsz + 1); /* { dg-warning "unterminated" } */ > +T (v0 ? b[0] : "", bsz + 1); > T (v0 ? b[1] : "", bsz + 1); > T (v0 ? b[2] : "", bsz + 1); > -T (v0 ? b[3] : "", bsz + 1); > +T (v0 ? b[3] : "", bsz + 1); /* { dg-warning "unterminated" } */ > > T (v0 ? "" : b[i0], bsz); > T (v0 ? "" : b[i1], bsz); > @@ -168,11 +168,11 @@ T (v0 ? b[i3] : "", bsz); > T (v0 ? "" : b[i0], bsz + 1); > T (v0 ? "" : b[i1], bsz + 1); > T (v0 ? "" : b[i2], bsz + 1); > -T (v0 ? "" : b[i3], bsz + 1); > +T (v0 ? "" : b[i3], bsz + 1); /* { dg-warning "unterminated" } */ > T (v0 ? b[i0] : "", bsz + 1); > T (v0 ? b[i1] : "", bsz + 1); > T (v0 ? b[i2] : "", bsz + 1); > -T (v0 ? b[i3] : "", bsz + 1); > +T (v0 ? b[i3] : "", bsz + 1); /* { dg-warning "unterminated" } */ > > T (v0 ? "1234" : b[3], bsz); > T (v0 ? "1234" : b[i3], bsz); > @@ -184,17 +184,18 @@ T (v0 ? b[0] : b[2], bsz); > T (v0 ? b[2] : b[3], bsz); > T (v0 ? b[3] : b[2], bsz); > > -T (v0 ? "1234" : b[3], bsz + 1); > -T (v0 ? "1234" : b[i3], bsz + 1); > -T (v0 ? b[3] : "1234", bsz + 1); > -T (v0 ? b[i3] : "1234", bsz + 1); > +/* New warnings being thrown after PR86937. */ > +T (v0 ? "1234" : b[3], bsz + 1); /* { dg-warning "unterminated" } */ > +T (v0 ? "1234" : b[i3], bsz + 1); /* { dg-warning "unterminated" } */ > +T (v0 ? b[3] : "1234", bsz + 1); /* { dg-warning "unterminated" } */ > +T (v0 ? b[i3] : "1234", bsz + 1); /* { dg-warning "unterminated" } */ > > /* That the following are not diagnosed is a bug/limitation resulting from > the fix for PR 103215. */ > -T (v0 ? a : b[3], bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" "pr103215" { xfail *-*-* } } */ > -T (v0 ? b[0] : b[2], bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" "pr103215" { xfail *-*-* } } */ > -T (v0 ? b[2] : b[3], bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" "pr103215" { xfail *-*-* } } */ > -T (v0 ? b[3] : b[2], bsz + 1); /* { dg-warning "bound 6 exceeds source > size 5" "pr103215" { xfail *-*-* } } */ > +T (v0 ? a : b[3], bsz + 1); > +T (v0 ? b[0] : b[2], bsz + 1); > +T (v0 ? b[2] : b[3], bsz + 1); /* { dg-warning "unterminated" } */ > +T (v0 ? b[3] : b[2], bsz + 1); /* { dg-warning "unterminated" } */ > > struct A { char a[5], b[5]; }; > > -- > 2.43.0 >
