ffi: preserve link register in ppc64 trampoline

The ppc64 fast FFI trampoline uses `bl` to obtain the address used to
load its target literal. This overwrites the caller's link register, so
optimized FFI calls return into the trampoline and loop indefinitely.
Save and restore the caller's link register around the branch. Also
align the target literal for both even and odd GP argument counts.
Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: codex:gpt-5.6-sol
PR-URL: #64792
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: trivikr

src/ffi/platforms/ppc64.cc

@@ -134,13 +134,21 @@ extern "C" bool node_ffi_create_fast_trampoline(
  }
 
 // Load the target address from the literal pool into r12, then branch through
-// CTR. ELFv2 functions can use r12 to establish their TOC on global entry.
-Emit32(&cursor, Bl(1)); // bl .+4
-Emit32(&cursor, Mfspr(12, 8)); // mflr r12
-Emit32(&cursor, Ld(12, 12, 20)); // ld r12, literal-mflr(r12)
-Emit32(&cursor, Mtspr(9, 12)); // mtctr r12
-Emit32(&cursor, 0x4e800420); // bctr
-Emit32(&cursor, 0x60000000); // nop; align literal to 8 bytes
+// CTR. Save the caller's link register before using `bl` to obtain the
+// trampoline's address, and restore it before the tail branch so the native
+// target returns to V8 rather than back into the trampoline. ELFv2 functions
+// can use r12 to establish their TOC on global entry.
+Emit32(&cursor, Mfspr(0, 8)); // mflr r0
+Emit32(&cursor, Bl(1)); // bl .+4
+Emit32(&cursor, Mfspr(12, 8)); // mflr r12
+Emit32(&cursor, Mtspr(8, 0)); // mtlr r0
+constunsigned literal_offset = gp_count % 2 == 0 ? 24 : 20;
+Emit32(&cursor, Ld(12, 12, literal_offset));
+Emit32(&cursor, Mtspr(9, 12)); // mtctr r12
+Emit32(&cursor, 0x4e800420); // bctr
+if (gp_count % 2 == 0) {
+Emit32(&cursor, 0x60000000); // nop; align literal to 8 bytes
+ }
 Emit64(&cursor, reinterpret_cast<uintptr_t>(target));
 
 constsize_t written = reinterpret_cast<uint8_t*>(cursor) -