[gcc(refs/vendors/ibm/heads/gcc-17-future)] Fix -mregnames issue with dense math registers.

https://gcc.gnu.org/g:6f925201a56dd8f27a1e371ffcfa074acf137d9d

commit 6f925201a56dd8f27a1e371ffcfa074acf137d9d
Author: Michael Meissner <[email protected]>
Date:   Thu Jul 2 20:05:58 2026 -0400

    Fix -mregnames issue with dense math registers.
    
    Kishan Parmar <[email protected]> issued a patch that made me re-examine the
    code I had put out for dense math registers.  This patch fixes two of these problems.
    
    The first problem is the output operand %A<n> was always printing out a number,
    even if -mregnames was used.  I fixed this by using reg_names[..] for dense math
    registers.  For power10/power11 accumulators that are overlaid on top of VSX
    registers, it just prints out the VSX register number divided by 4.
    
    The second problem is once %A<n> can print a register name instead of a number,
    the next problem is if you use -mregnames, GCC would put out %dmr<n> while the
    assembler wants %dm<n>.  I fixed this to use the correct name.
    
    I have committed all of the patches in my backlog (dense math registers, other
    -mcpu=future instructions, random bug fixes, support for _Float16 and
    __bfloat16, and optimizations for vector logical operations on power10/power11)
    into the IBM vendor branch:
    
            vendors/ibm/gcc-17-future
    
    I have tested these patches on both big endian and little endian PowerPC
    servers, with no regressions.  Can I check these patches into the trunk?
    
    2026-07-02  Michael Meissner  <[email protected]>
    
    gcc/
    
            * config/rs6000/rs6000.cc (alt_reg_names): Use %dm<n> for dense math
            registers, not %dmr<n>.
            (print_operand): Add a comment for %A<n>, of why we can't emit %dm<n>
            since the assembler doesn't recognize %dm0 in this context.

Diff:
---
 gcc/config/rs6000/rs6000.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2d983fee0b56..dd66d190a9d7 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -1259,7 +1259,7 @@ static const char alt_reg_names[][8] =
   /* vrsave vscr sfp */
   "vrsave", "vscr", "sfp",
   /* dense math registers.  */
-  "%dmr0", "%dmr1", "%dmr2", "%dmr3", "%dmr4", "%dmr5", "%dmr6", "%dmr7",
+  "%dm0", "%dm1", "%dm2", "%dm3", "%dm4", "%dm5", "%dm6", "%dm7",
 };
 #endif
 
@@ -14489,6 +14489,10 @@ print_operand (FILE *file, rtx x, int code)
 	 accumulators overlapping with the FPR registers.  */
       if (!REG_P (x))
 	output_operand_lossage ("invalid %%A value");
+
+      /* We can't use reg_names[REGNO (x)] here because the assembler doesn't
+	 recognize things list dmxvf64gerpp %dm6,%vs32,%vs40.  We continue to
+	 just put out a number 0..7.  */
       else if (TARGET_DENSE_MATH && DMF_REGNO_P (REGNO (x)))
 	fprintf (file, "%d", REGNO (x) - FIRST_DMF_REGNO);
       else if (!FP_REGNO_P (REGNO (x)) || (REGNO (x) % 4) != 0)