Re: [PATCH 2/2] middle-end/126788 - more vector folding

On Fri, Aug 14, 2026 at 8:13 PM Richard Biener <[email protected]> wrote:
>
> The last match.pd update for double-conversion folding didn't handle
> the case where a vector conversion would go away because
> supportable_convert_operation doesn't consider a noop conversion
> (or a sign conversion).  The following rectifies this, allowing
> a NOP_EXPR and VIEW_CONVERT_EXPR for same mode types as supportable
> conversion as we can RTL expand that just fine.
>
> This shows that the vectorizer routed sign-conversions through
> vectorizable_assignment but now vectorizable_conversion would
> handle it, emitting NOP_EXPRs (which is fine) instead of
> VIEW_CONVERT_EXPRs (which we declared canonical for vectors).
> This would confuse some foldings, leading to testsuite FAILs,
> so make vectorizable_conversion also prefer VIEW_CONVERT_EXPRs
> here.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> I'll wait on comments on 1/2 before pushing.
>
> Richard.
>
>         PR middle-end/126788
>         * optabs-tree.cc (supportable_convert_operation): For same
>         modes allow NOP_EXPR and VIEW_CONVERT_EXPR.
>         * tree-ssa-forwprop.cc (simplify_vector_constructor): Avoid
>         converting the vector to be used for blending into the
>         result to the type of the permutation.
>         * tree-vect-stmts.cc (vectorizable_conversion): Prefer
>         VIEW_CONVERT_EXPR for sign changes.
>
>         * gcc.target/i386/pr126788.c: New testcase.
> ---
>  gcc/optabs-tree.cc                       |  4 ++++
>  gcc/testsuite/gcc.target/i386/pr126788.c | 24 ++++++++++++++++++++++++
>  gcc/tree-ssa-forwprop.cc                 |  1 +
>  gcc/tree-vect-stmts.cc                   |  9 ++++++++-
>  4 files changed, 37 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.target/i386/pr126788.c
>

It failed with this:

[hjl@gnu-zen4-1 gcc]$ cat /tmp/x.C
extern int _M_current;
extern int end ();
struct color
{
  char r;
  struct
  {
    unsigned char g;
    unsigned char b;
  } m_24bit;
  color (char g, char b)
  {
    m_24bit.g = g;
    m_24bit.b = b;
  }
};
void
set_style_bg_color (color)
{
  for (;;)
    {
      const unsigned char g = _M_current;
      if (end ())
break;
      set_style_bg_color (color (g, _M_current));
    }
}
[hjl@gnu-zen4-1 gcc]$ ./xgcc -B./ -S -O2 -march=x86-64-v4 /tmp/x.C
/tmp/x.C: In function ‘void set_style_bg_color(color)’:
/tmp/x.C:18:1: error: type mismatch between an SSA_NAME and its symbol
   18 | set_style_bg_color (color)
      | ^~~~~~~~~~~~~~~~~~
/tmp/x.C:18:1: error: type mismatch between an SSA_NAME and its symbol
while verifying SSA_NAME vect_g_49.10_100 in statement
vect_g_49.10_100 = (vector(2) unsigned char) _101;
during GIMPLE pass: slp
/tmp/x.C:18:1: internal compiler error: verify_ssa failed
0x4fa8116 internal_error(char const*, ...)
/export/gnu/import/git/gitlab/x86-gcc-test/gcc/diagnostic-global-context.cc:787
0x313c855 verify_ssa(bool, bool)
/export/gnu/import/git/gitlab/x86-gcc-test/gcc/tree-ssa.cc:1226
0x2c64811 execute_function_todo
/export/gnu/import/git/gitlab/x86-gcc-test/gcc/passes.cc:2107
0x2c6345a do_per_function
/export/gnu/import/git/gitlab/x86-gcc-test/gcc/passes.cc:1699
0x2c649a8 execute_todo
/export/gnu/import/git/gitlab/x86-gcc-test/gcc/passes.cc:2152
./cc1plus -quiet -iprefix
/export/build/gnu/tools-build/gcc-gitlab-test-debug/build-x86_64-linux/gcc/../lib/gcc/x86_64-pc-linux-gnu/17.0.0/
-isystem ./include -isystem ./include-fixed -D_GNU_SOURCE /tmp/x.C
-quiet -dumpbase x.C -dumpbase-ext .C -march=x86-64-v4
-mtls-dialect=gnu2 -O2 -o x.s
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
[hjl@gnu-zen4-1 gcc]$

-- 
H.J.