[gcc r11-8146] simplify-rtx: Punt on simplify_{, gen_}subreg to IBM double double if bits are lost [PR99648]

[gcc r11-8146] simplify-rtx: Punt on simplify_{, gen_}subreg to IBM double double if bits are lost [PR99648]

[Jakub Jelinek]

https://gcc.gnu.org/g:c0f772894b6b3cd8ed5c5dd09d0c7917f51cf70f

commit r11-8146-gc0f772894b6b3cd8ed5c5dd09d0c7917f51cf70f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 13 09:41:20 2021 +0200

    simplify-rtx: Punt on simplify_{,gen_}subreg to IBM double double if bits are lost [PR99648]
    
    Similarly to PR95450 done on GIMPLE, this patch punts if we try to
    simplify_{gen_,}subreg from some constant into the IBM double double
    IFmode (or sometimes TFmode) if the double double format wouldn't preserve
    the bits.  Not all values are valid in IBM double double, e.g. the format
    requires that the upper double is the whole value rounded to double, and
    if in some cases such as in the pr71522.c testcase with -m32 -Os -mcpu=power7
    some non-floating data is copied through long double variable, we can
    simplify a subreg into something that has different value.
    
    Fixed by punting if the planned simplify_immed_subreg result doesn't
    encode to bitwise identical values compared to what we were decoding.
    
    As for the simplify_gen_subreg change, I think it would be desirable
    to just avoid creating SUBREGs of constants on all targets and for all
    constants, if simplify_immed_subreg simplified, fine, otherwise punt,
    but as we are late in GCC11 development, the patch instead guards this
    behavior on MODE_COMPOSITE_P (outermode) - i.e. only conversions to
    powerpc{,64,64le} double double long double - and only for the cases where
    simplify_immed_subreg was called.
    
    2021-04-13  Jakub Jelinek  <jakub@redhat.com>
    
            PR target/99648
            * simplify-rtx.c (simplify_immed_subreg): For MODE_COMPOSITE_P
            outermode, return NULL if the result doesn't encode back to the
            original byte sequence.
            (simplify_gen_subreg): Don't create SUBREGs from constants to
            MODE_COMPOSITE_P outermode.

Diff:
---
 gcc/simplify-rtx.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index 571e2337e27..d13c390a20c 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -7035,12 +7035,19 @@ simplify_immed_subreg (fixed_size_mode outermode, rtx x,
       while (buffer.length () < buffer_bytes)
 	buffer.quick_push (filler);
     }
-  else
+  else if (!native_encode_rtx (innermode, x, buffer, first_byte, inner_bytes))
+    return NULL_RTX;
+  rtx ret = native_decode_rtx (outermode, buffer, 0);
+  if (ret && MODE_COMPOSITE_P (outermode))
     {
-      if (!native_encode_rtx (innermode, x, buffer, first_byte, inner_bytes))
+      auto_vec<target_unit, 128> buffer2 (buffer_bytes);
+      if (!native_encode_rtx (outermode, ret, buffer2, 0, buffer_bytes))
 	return NULL_RTX;
-      }
-  return native_decode_rtx (outermode, buffer, 0);
+      for (unsigned int i = 0; i < buffer_bytes; ++i)
+	if (buffer[i] != buffer2[i])
+	  return NULL_RTX;
+    }
+  return ret;
 }
 
 /* Simplify SUBREG:OUTERMODE(OP:INNERMODE, BYTE)
@@ -7336,6 +7343,13 @@ simplify_context::simplify_gen_subreg (machine_mode outermode, rtx op,
       || GET_MODE (op) == VOIDmode)
     return NULL_RTX;
 
+  if (MODE_COMPOSITE_P (outermode)
+      && (CONST_SCALAR_INT_P (op)
+	  || CONST_DOUBLE_AS_FLOAT_P (op)
+	  || CONST_FIXED_P (op)
+	  || GET_CODE (op) == CONST_VECTOR))
+    return NULL_RTX;
+
   if (validate_subreg (outermode, innermode, op, byte))
     return gen_rtx_SUBREG (outermode, op, byte);