PR target/114187: Fix ?Fmode SUBREG simplification in simplify_subreg.

This patch fixes PR target/114187 a typo/missed-optimization in simplify-rtx
that's exposed by (my) changes to x86_64's parameter passing.  The context
is that construction of double word (TImode) values now uses the idiom:

(ior:TI (ashift:TI (zero_extend:TI (reg:DI x)) (const_int 64 [0x40]))
        (zero_extend:TI (reg:DI y)))

Extracting the DImode highpart and lowpart halves of this complex expression
is supported by simplications in simplify_subreg.  The problem is when the
doubleword TImode value represents two DFmode fields, there isn't a direct
simplification to extract the highpart or lowpart SUBREGs, but instead GCC
uses two steps, extract the DImode {high,low} part and then cast the result
back to a floating point mode, DFmode.

The (buggy) code to do this is:

  /* If the outer mode is not integral, try taking a subreg with the equivalent
     integer outer mode and then bitcasting the result.
     Other simplifications rely on integer to integer subregs and we'd
     potentially miss out on optimizations otherwise.  */
  if (known_gt (GET_MODE_SIZE (innermode),
                GET_MODE_SIZE (outermode))
      && SCALAR_INT_MODE_P (innermode)
      && !SCALAR_INT_MODE_P (outermode)
      && int_mode_for_size (GET_MODE_BITSIZE (outermode),
                            0).exists (&int_outermode))
    {
      rtx tem = simplify_subreg (int_outermode, op, innermode, byte);
      if (tem)
        return simplify_gen_subreg (outermode, tem, int_outermode, byte);
    }

The issue/mistake is that the second call, to simplify_subreg, shouldn't
use "byte" as the final argument; the offset has already been handled by
the first call, to simplify_subreg, and this second call is just a type
conversion from an integer mode to floating point (from DImode to DFmode).

Interestingly, this mistake was already spotted by Richard Sandiford when
the optimization was originally contributed in January 2023.
https://gcc.gnu.org/pipermail/gcc-patches/2023-January/610920.html
>> Richard Sandiford writes:
>> Also, the final line should pass 0 rather than byte.

Unfortunately a miscommunication/misunderstanding in a later thread
https://gcc.gnu.org/pipermail/gcc-patches/2023-February/612898.html
resulted in this correction being undone.  Using lowpart_subreg should
avoid/reduce confusion in future.

2024-03-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/114187
	* simplify-rtx.cc (simplify_context::simplify_subreg): Call
	lowpart_subreg to perform type conversion, to avoid confusion
	over the offset to use in the call to simplify_reg_subreg.

gcc/testsuite/ChangeLog
	PR target/114187
	* g++.target/i386/pr114187.C: New test case.

0 files changed, 0 insertions, 0 deletions