Explain why we don't make use of RDPC in the sparc PIC register setup.

* sysdeps/unix/sparc/sysdep.h: Document why we don't use "rd %pc" in the PIC register setup sequences.
git-mirror · Feb 21, 2012 · 0722d7c · 0722d7c
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diff --git a/ChangeLog b/ChangeLog
@@ -1,5 +1,8 @@
 2012-02-20  David S. Miller  <davem@davemloft.net>
 
+	* sysdeps/unix/sparc/sysdep.h: Document why we don't use
+	"rd %pc" in the PIC register setup sequences.
+
 	* sysdeps/sparc/crti.S: Try to use GOTDATA relocs.
 	* sysdeps/sparc/sparc32/dl-machine.h (RTLD_START): Likewise.
 	* sysdeps/sparc/sparc32/elf/start.S: Likewise.

diff --git a/sysdeps/unix/sparc/sysdep.h b/sysdeps/unix/sparc/sysdep.h
@@ -39,6 +39,9 @@ __sparc_get_pc_thunk.reg:		   				\
 	.previous;							\
 	.endif;
 
+/* Even when v9 we use a call sequence instead of using "rd %pc" because
+   RDPC is extremely expensive and incurs a full pipeline flush.  */
+
 #define SETUP_PIC_REG(reg)						\
 	SPARC_PIC_THUNK(reg)						\
 	sethi	%hi(_GLOBAL_OFFSET_TABLE_-4), %##reg;			\