On Wed, Jun 2, 2021 at 10:55 PM Victor Tong <vit...@microsoft.com> wrote: > > Hi Richard, > > Thanks for reviewing my patch. I did a search online and you're right -- > there isn't a vector modulo instruction. I'll remove the X * (Y / X) --> Y - > (Y % X) pattern and the existing X - (X / Y) * Y --> X % Y from triggering on > vector types. > > I looked into why the following pattern isn't triggering: > > (simplify > (minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) > (view_convert @1)) > > The nop_converts expand into tree_nop_conversion_p checks. In fn2() of the > testsuite/gcc.dg/fold-minus-6.c, the expression during generic matching looks > like: > > 42 - (long int) (42 - 42 % x) > > When looking at the right-hand side of the expression (the (long int) (42 - > 42 % x)), the tree_nop_conversion_p check fails because of the type precision > difference. The expression inside of the cast has a 32-bit precision and the > outer expression has a 64-bit precision. > > I looked around at other patterns and it seems like nop_convert and > view_convert are used because of underflow/overflow concerns. I'm not > familiar with the two constructs. What's the difference between using them > and checking TYPE_OVERFLOW_UNDEFINED? In the scenario above, since > TYPE_OVERFLOW_UNDEFINED is true, the second pattern that I added (X - (X - Y) > --> Y) gets triggered.
But TYPE_OVERFLOW_UNDEFINED is not a good condition here since the conversion is the problematic one and conversions have implementation defined behavior. Now, the above does not match because it wasn't designed to, and for non-constant '42' it would have needed a (convert ...) around the first @0 as well (matching of constants is by value, not by value + type). That said, your +/* X - (X - Y) --> Y */ +(simplify + (minus (convert1? @0) (convert2? (minus @@0 @1))) + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && TYPE_OVERFLOW_UNDEFINED(type)) + (convert @1))) would match (int)x - (int)(x - y) where you assert the outer subtract has undefined behavior on overflow but the inner subtract could wrap and the (int) conversion can be truncating or widening. Is that really always a valid transform then? Richard. > Thanks, > Victor > > > From: Richard Biener <richard.guent...@gmail.com> > Sent: Tuesday, April 27, 2021 1:29 AM > To: Victor Tong <vit...@microsoft.com> > Cc: gcc-patches@gcc.gnu.org <gcc-patches@gcc.gnu.org> > Subject: [EXTERNAL] Re: [PATCH] tree-optimization: Optimize division followed > by multiply [PR95176] > > On Thu, Apr 1, 2021 at 1:03 AM Victor Tong via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > Hello, > > > > This patch fixes PR tree-optimization/95176. A new pattern in match.pd was > > added to transform "a * (b / a)" --> "b - (b % a)". A new test case was > > also added to cover this scenario. > > > > The new pattern interfered with the existing pattern of "X - (X / Y) * Y". > > In some cases (such as in fn4() in gcc/testsuite/gcc.dg/fold-minus-6.c), > > the new pattern is applied causing the existing pattern to no longer apply. > > This results in worse code generation because the expression is left as "X > > - (X - Y)". An additional subtraction pattern of "X - (X - Y) --> Y" was > > added to this patch to avoid this regression. > > > > I also didn't remove the existing pattern because it triggered in more > > cases than the new pattern because of a tree_invariant_p check that's > > inserted by genmatch for the new pattern. > > Yes, we do not handle using Y multiple times when it might contain > side-effects in GENERIC folding > (comments in genmatch suggest we can use save_expr but we don't > implement this [anymore]). > > On GIMPLE there's also the issue that your new pattern creates a > complex expression which > makes it failed to be used by value-numbering for example where the > old pattern was OK > (eventually, if no conversion was required). > > So indeed it looks OK to preserve both. > > I wonder why you needed the > > +/* X - (X - Y) --> Y */ > +(simplify > + (minus (convert1? @0) (convert2? (minus @@0 @1))) > + (if ((INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) && > TYPE_OVERFLOW_UNDEFINED(type)) > + (convert @1))) > > pattern since it should be handled by > > /* Match patterns that allow contracting a plus-minus pair > irrespective of overflow issues. */ > /* (A +- B) - A -> +- B */ > /* (A +- B) -+ B -> A */ > /* A - (A +- B) -> -+ B */ > /* A +- (B -+ A) -> +- B */ > > in particular > > (simplify > (minus @0 (nop_convert1? (minus (nop_convert2? @0) @1))) > (view_convert @1)) > > if there's supported cases missing I'd rather extend this pattern than > replicating it. > > +/* X * (Y / X) is the same as Y - (Y % X). */ > +(simplify > + (mult:c (convert1? @0) (convert2? (trunc_div @1 @@0))) > + (if (INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type)) > + (minus (convert @1) (convert (trunc_mod @1 @0))))) > > note that if you're allowing vector types you have to use > (view_convert ...) in the > transform and you also need to make sure that the target can expand > the modulo - I suspect that's an issue with the existing pattern as well. > I don't know of any vector ISA that supports modulo (or integer > division, that is). > Restricting the patterns to integer types is probably the most > sensible solution. > > Thanks, > Richard. > > > I verified that all "make -k check" tests pass when targeting > > x86_64-pc-linux-gnu. > > > > 2021-03-31 Victor Tong <vit...@microsoft.com> > > > > gcc/ChangeLog: > > > > * match.pd: Two new patterns: One to optimize division followed by > > multiply and the other to avoid a regression as explained above > > > > gcc/testsuite/ChangeLog: > > > > * gcc.dg/tree-ssa/20030807-10.c: Update existing test to look for a > > subtraction because a shift is no longer emitted > > * gcc.dg/pr95176.c: New test to cover optimizing division followed > > by multiply > > > > I don't have write access to the GCC repo but I've completed the FSF > > paperwork as I plan to make more contributions in the future. I'm looking > > for a sponsorship from an existing GCC maintainer before applying for write > > access. > > > > Thanks, > > Victor
