> -----Original Message-----
> From: Richard Biener <[email protected]>
> Sent: 23 February 2026 12:36
> To: Tamar Christina <[email protected]>
> Cc: [email protected]; nd <[email protected]>; [email protected]
> Subject: RE: [PATCH][vect]: enforce trapping math checks when early breaks
> [PR124142]
>
> On Mon, 23 Feb 2026, Tamar Christina wrote:
>
> > > -----Original Message-----
> > > From: Richard Biener <[email protected]>
> > > Sent: 23 February 2026 12:07
> > > To: Tamar Christina <[email protected]>
> > > Cc: [email protected]; nd <[email protected]>; [email protected]
> > > Subject: Re: [PATCH][vect]: enforce trapping math checks when early
> breaks
> > > [PR124142]
> > >
> > > On Mon, 23 Feb 2026, Tamar Christina wrote:
> > >
> > > > The loop
> > > >
> > > > void
> > > > __attribute__((noipa))
> > > > bug (int f, int *w, int l)
> > > > {
> > > > int i;
> > > > for (i = 0; i < l; ++i)
> > > > while (f % w[i]) w[i]--;
> > > > }
> > > >
> > > > is an uncounted loop which performs a trapping operation during
> > > vectorization.
> > > > Normally the vectorizer doesn't stop vectorization if the operation
> > > > can't
> be
> > > > masked (though ifcvt does).
> > > >
> > > > For Early breaks however this is unsafe as we are introducing a trap
> where
> > > the
> > > > original scalar code may not have.
> > > >
> > > > reductions however don't require masking when the scalar epilogue is
> used
> > > to
> > > > restart the last iteration as we use the previous value of the vector
> > > > in that
> > > > case and don't reduce "inactive" lanes. This would need to be adjusted
> > > when we
> > > > support staying inside the vector loop directly.
> > > >
> > > > Some tests now fail to vectorize (including some tsvc ones) which I
> > > > could
> > > have
> > > > xfail'ed but instead decided to add -fno-trapping-math just to get the
> > > > additional coverage they provide.
> > > >
> > > > Bootstrapped Regtested on aarch64-none-linux-gnu,
> > > > arm-none-linux-gnueabihf, x86_64-pc-linux-gnu
> > > > -m32, -m64 and no issues.
> > > >
> > > > Any objections?
> > >
> > > I was expecting the check to be in vect_analyze_early_break_dependences
> > > since I think there is a special thing about early break vectorization
> > > when a trapping operation is before the early exit condition or does
> > > feed it, like for the testcase at hand. Those operations cannot be
> > > masked since the very mask is not yet computed.
> >
> > I see your point.
> >
> > operations before the early break condition but not part of the condition
> > are also not valid. E.g.
> >
> > b = a / d;
> > if (a > c)
> > break;
> >
> > f[i] = b;
> >
> > because we'd be performing the division for iterations that may not have
> > happened for the scalar loop.
> >
> > So the only "safe" trapping operations are those between the last early
> > exit and the latch exit.
> >
> > For them to be safe they would have to be moved similar to stores.
> >
> > I can move the check there, but I'd leave the trap stmt motion for now..
>
> In particular a possibly trapping operation or call _after_ the
> early break condition should be OK if we exit the loop and defer
> the last iteration to the epilog (once we stop that, those need to
> be masked of course).
Yeah, I was just saying that in the above example, there's no reason we couldn't
move b until after all early break statements and still be able to vectorize
correctly because b isn't actually needed in that BB and can be sunk.
>
> Your patch does not catch the case where the trapping stmt is
> before the early exit but we can use partial vectors - the mask
> would be wrong (from the main IV) at that point, no?
Definitely, I wasn't arguing that. I was just saying that the above case where
b could be sunk is valid after sinking, that I'd like to punt that particular
optimization
to the next release and for now reject the case above.
That is to say, are you ok with just rejecting the case above (where a trapping
statement that's not part of the break condition and could be sunk and made
valid)
Or did you want me to actually try to handle the sinking now.
Thanks,
Tamar
>
> Richard.
>
> >
> > Thanks,
> > Tamar
> >
> > >
> > > There might be other latent issues with trapping operations but I'd
> > > rather split those out.
> > >
> > > So, am I missing something?
> > >
> > > Thanks,
> > > Richard.
> > >
> > > > Thanks,
> > > > Tamar
> > > >
> > > > gcc/ChangeLog:
> > > >
> > > > PR tree-optimization/124142
> > > > * tree-vect-stmts.cc (vectorizable_call,
> > > > vectorizable_operation):
> > > > For early break require masking when operation can trap.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > PR tree-optimization/124142
> > > > * gcc.dg/vect/tsvc/vect-tsvc-s481.c: Add -fno-trapping-math.
> > > > * gcc.dg/vect/tsvc/vect-tsvc-s482.c: Likewise.
> > > > * gcc.dg/vect/vect-early-break_61.c: Likewise.
> > > > * gcc.target/aarch64/vect-early-break-cbranch_3.c: Likewise.
> > > > * gcc.dg/vect/vect-early-break_143-pr124142.c: New test.
> > > >
> > > > ---
> > > > diff --git a/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s481.c
> > > b/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s481.c
> > > > index
> > >
> e4433385d6686806a75fffe22f90e3bfb603564c..23c9961691d2f0c952894
> > > 9c5c44dae3e264dc3ec 100644
> > > > --- a/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s481.c
> > > > +++ b/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s481.c
> > > > @@ -1,7 +1,7 @@
> > > > /* This file is distributed under the University of Illinois Open
> > > > Source
> > > > License. See license.txt for details. */
> > > >
> > > > -/* { dg-additional-options "--param vect-epilogues-nomask=0" } */
> > > > +/* { dg-additional-options "--param vect-epilogues-nomask=0 -fno-
> > > trapping-math" } */
> > > > /* { dg-require-effective-target vect_float } */
> > > > /* { dg-require-effective-target vect_early_break_hw } */
> > > > /* { dg-add-options vect_early_break } */
> > > > @@ -41,4 +41,4 @@ int main (int argc, char **argv)
> > > > return 0;
> > > > }
> > > >
> > > > -/* { dg-final { scan-tree-dump "vectorized 1 loops" "vect" { xfail { !
> > > vect_early_break} } } } */
> > > > +/* { dg-final { scan-tree-dump "vectorized 1 loops" "vect" { xfail { !
> > > vect_early_break } } } } */
> > > > diff --git a/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s482.c
> > > b/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s482.c
> > > > index
> > >
> 146df409ecc64e9535583c0d0083469d7aa24031..38426845a3d115432bfc
> > > 68d9e1e7539fe10e27e2 100644
> > > > --- a/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s482.c
> > > > +++ b/gcc/testsuite/gcc.dg/vect/tsvc/vect-tsvc-s482.c
> > > > @@ -1,7 +1,7 @@
> > > > /* This file is distributed under the University of Illinois Open
> > > > Source
> > > > License. See license.txt for details. */
> > > >
> > > > -/* { dg-additional-options "--param vect-epilogues-nomask=0" } */
> > > > +/* { dg-additional-options "--param vect-epilogues-nomask=0 -fno-
> > > trapping-math" } */
> > > > /* { dg-require-effective-target vect_float } */
> > > > /* { dg-require-effective-target vect_early_break_hw } */
> > > > /* { dg-add-options vect_early_break } */
> > > > diff --git a/gcc/testsuite/gcc.dg/vect/vect-early-break_143-pr124142.c
> > > b/gcc/testsuite/gcc.dg/vect/vect-early-break_143-pr124142.c
> > > > new file mode 100644
> > > > index
> > >
> 0000000000000000000000000000000000000000..b9141e3f15a9d15211
> > > 8c2d8e84fc45d5e83066cb
> > > > --- /dev/null
> > > > +++ b/gcc/testsuite/gcc.dg/vect/vect-early-break_143-pr124142.c
> > > > @@ -0,0 +1,16 @@
> > > > +/* { dg-do compile } */
> > > > +/* { dg-add-options vect_early_break } */
> > > > +/* { dg-require-effective-target vect_early_break } */
> > > > +/* { dg-require-effective-target vect_int } */
> > > > +
> > > > +/* { dg-final { scan-tree-dump-not "LOOP VECTORIZED" "vect" } } */
> > > > +
> > > > +void
> > > > +__attribute__((noipa))
> > > > +bug (int f, int *w, int l)
> > > > +{
> > > > + int i;
> > > > + for (i = 0; i < l; ++i)
> > > > + while (f % w[i]) w[i]--;
> > > > +}
> > > > +
> > > > diff --git a/gcc/testsuite/gcc.dg/vect/vect-early-break_61.c
> > > b/gcc/testsuite/gcc.dg/vect/vect-early-break_61.c
> > > > index
> > >
> c789ec01f32c6b958c6a3663531a7b7517b94477..0cd20549cdc656baad2b
> > > 5561e39ca999bca98bb0 100644
> > > > --- a/gcc/testsuite/gcc.dg/vect/vect-early-break_61.c
> > > > +++ b/gcc/testsuite/gcc.dg/vect/vect-early-break_61.c
> > > > @@ -2,6 +2,7 @@
> > > > /* { dg-do compile } */
> > > > /* { dg-require-effective-target vect_early_break } */
> > > > /* { dg-require-effective-target vect_float } */
> > > > +/* { dg-additional-options "-fno-trapping-math" } */
> > > >
> > > > typedef float real_t;
> > > > __attribute__((aligned(64))) real_t a[32000], b[32000], c[32000];
> > > > diff --git a/gcc/testsuite/gcc.target/aarch64/vect-early-break-
> cbranch_3.c
> > > b/gcc/testsuite/gcc.target/aarch64/vect-early-break-cbranch_3.c
> > > > index
> > >
> 8980b9f04f9a15ded9e90954e25b2c0578681761..b252b80ef291efd4f6fc9
> > > 8224a44ef7fadc68d6c 100644
> > > > --- a/gcc/testsuite/gcc.target/aarch64/vect-early-break-cbranch_3.c
> > > > +++ b/gcc/testsuite/gcc.target/aarch64/vect-early-break-cbranch_3.c
> > > > @@ -1,5 +1,5 @@
> > > > /* { dg-do compile } */
> > > > -/* { dg-options "-O3 -fno-schedule-insns -fno-reorder-blocks -fno-
> > > schedule-insns2 --param aarch64-autovec-preference=asimd-only" } */
> > > > +/* { dg-options "-O3 -fno-trapping-math -fno-schedule-insns -fno-
> reorder-
> > > blocks -fno-schedule-insns2 --param aarch64-autovec-preference=asimd-
> > > only" } */
> > > > /* { dg-final { check-function-bodies "**" "" "" { target lp64 } } } */
> > > >
> > > > #pragma GCC target "+sve"
> > > > diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> > > > index
> > >
> 22285250aa8d4f600721c647a83d1b2bafe7ef2a..cb7217ad6f92ad4b2bd54
> > > 3733c802606f3fba03e 100644
> > > > --- a/gcc/tree-vect-stmts.cc
> > > > +++ b/gcc/tree-vect-stmts.cc
> > > > @@ -3650,6 +3650,18 @@ vectorizable_call (vec_info *vinfo,
> > > > LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
> > > > }
> > > > }
> > > > +
> > > > + /* If the operation traps, and we're an early break loop then
> > > > don't
> allow
> > > > + vectorization if masking isn't supported. */
> > > > + if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > > + {
> > > > + if (dump_enabled_p ())
> > > > + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> > > > + "can't use a fully-masked loop and loop
> > > > has "
> > > > + "multiple exits. Cannot vectorize as
> > > > operation "
> > > > + "may trap.\n");
> > > > + return false;
> > > > + }
> > > > }
> > > >
> > > > /* If that fails, try asking for a target-specific built-in
> > > > function. */
> > > > @@ -6792,7 +6804,8 @@ vectorizable_operation (vec_info *vinfo,
> > > > Similarly, if this operation is part of a reduction, a
> > > > fully-masked
> > > > loop should only change the active lanes of the reduction chain,
> > > > keeping the inactive lanes as-is. */
> > > > - bool mask_out_inactive = ((!is_invariant && gimple_could_trap_p
> (stmt))
> > > > + bool could_trap_p = gimple_could_trap_p (stmt);
> > > > + bool mask_out_inactive = ((!is_invariant && could_trap_p)
> > > > || reduc_idx >= 0);
> > > >
> > > > if (cost_vec) /* transformation not required. */
> > > > @@ -6821,6 +6834,23 @@ vectorizable_operation (vec_info *vinfo,
> > > > }
> > > > }
> > > >
> > > > + /* If the operation traps, and we're an early break loop then
> > > > don't
> allow
> > > > + vectorization if masking isn't supported. Reductions are OK
> > > > because if
> > > > + we take an early exit, the epilog will get the value of the
> > > > previous
> > > > + iterations and we recompute the remainder. */
> > > > + if (loop_vinfo
> > > > + && could_trap_p
> > > > + && !LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo)
> > > > + && LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > > + {
> > > > + if (dump_enabled_p ())
> > > > + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> > > > + "can't use a fully-masked loop and loop
> > > > has "
> > > > + "multiple exits. Cannot vectorize as
> > > > operation "
> > > > + "may trap.\n");
> > > > + return false;
> > > > + }
> > > > +
> > > > /* Put types on constant and invariant SLP children. */
> > > > if (!vect_maybe_update_slp_op_vectype (slp_op0, vectype)
> > > > || !vect_maybe_update_slp_op_vectype (slp_op1, vectype)
> > > >
> > > >
> > > >
> > >
> > > --
> > > Richard Biener <[email protected]>
> > > SUSE Software Solutions Germany GmbH,
> > > Frankenstrasse 146, 90461 Nuernberg, Germany;
> > > GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG
> > > Nuernberg)
> >
>
> --
> Richard Biener <[email protected]>
> SUSE Software Solutions Germany GmbH,
> Frankenstrasse 146, 90461 Nuernberg, Germany;
> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG
> Nuernberg)
