[gcc r16-3679] c++: Fix mangling of _Float16 template args [PR121801]

[Matthias Kretz]

https://gcc.gnu.org/g:19d1c7c28f4fd0557dd868a7a4041b00ceada890

commit r16-3679-g19d1c7c28f4fd0557dd868a7a4041b00ceada890
Author: Matthias Kretz <m.kretz@gsi.de>
Date:   Fri Sep 5 12:16:34 2025 +0200

    c++: Fix mangling of _Float16 template args [PR121801]
    
    Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
    
    gcc/testsuite/ChangeLog:
    
            PR c++/121801
            * g++.dg/abi/pr121801.C: New test.
    
    gcc/cp/ChangeLog:
    
            PR c++/121801
            * mangle.cc (write_real_cst): Handle 16-bit real and assert
            that reals have 16 bits or a multiple of 32 bits.

Diff:
---
 gcc/cp/mangle.cc                    | 15 ++++++++++++++-
 gcc/testsuite/g++.dg/abi/pr121801.C | 13 +++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 80be40d2dce3..c1aae2de52b0 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -2176,11 +2176,24 @@ write_real_cst (const tree value)
   int i, limit, dir;
 
   tree type = TREE_TYPE (value);
-  int words = GET_MODE_BITSIZE (SCALAR_FLOAT_TYPE_MODE (type)) / 32;
+  int bits = GET_MODE_BITSIZE (SCALAR_FLOAT_TYPE_MODE (type));
+  int words = bits / 32;
 
   real_to_target (target_real, &TREE_REAL_CST (value),
 		  TYPE_MODE (type));
 
+  if (words == 0)
+    {
+      /* _Float16 and std::bfloat16_t are the only supported types smaller than
+         32 bits.  */
+      gcc_assert (bits == 16);
+      sprintf (buffer, "%04lx", (unsigned long) target_real[0]);
+      write_chars (buffer, 4);
+      return;
+    }
+
+  gcc_assert (bits % 32 == 0);
+
   /* The value in target_real is in the target word order,
      so we must write it out backward if that happens to be
      little-endian.  write_number cannot be used, it will
diff --git a/gcc/testsuite/g++.dg/abi/pr121801.C b/gcc/testsuite/g++.dg/abi/pr121801.C
new file mode 100644
index 000000000000..cd35186d8881
--- /dev/null
+++ b/gcc/testsuite/g++.dg/abi/pr121801.C
@@ -0,0 +1,13 @@
+// PR c++/121801
+// { dg-do compile { target { c++20 && float16 } } }
+// { dg-add-options float16 }
+
+template<_Float16 T> void f() {}
+
+void uses() {
+  f<_Float16(1)>();
+  f<_Float16(2)>();
+}
+
+// { dg-final { scan-assembler "_Z1fILDF16_3c00EEvv" } }
+// { dg-final { scan-assembler "_Z1fILDF16_4000EEvv" } }