[PATCH] Don't count asm goto for 4 branch limit optimization (PR target/59625)

Jakub Jelinek jakub@redhat.com
Fri Jan 3 08:37:00 GMT 2014


Hi!

Linus complained about padding inserted in between asm goto.  In 4.8
even -mtune=generic performs ix86_avoid_jump_mispredicts, in 4.9 only e.g.
-mtune=atom and others, but not generic.
The issue is that assuming every asm goto must contain a jump is too
pessimistic, often asm goto (may) transfer control to the label(s) through
other means (e.g. on the side data structure with labels etc.), plus we
conservatively assume asm goto is 0 bytes long, so we were adding the
paddings pretty much always when multiple asm goto stmts appeared
consecutively.

This patch just ignores asm goto from the optimization.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and 4.8
after a while?

2014-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/59625
	* config/i386/i386.c (ix86_avoid_jump_mispredicts): Don't consider
	asm goto as jump.

	* gcc.target/i386/pr59625.c: New test.

--- gcc/config/i386/i386.c.jj	2014-01-01 00:45:15.000000000 +0100
+++ gcc/config/i386/i386.c	2014-01-02 14:44:07.980841529 +0100
@@ -38852,7 +38852,8 @@ ix86_avoid_jump_mispredicts (void)
 	      while (nbytes + max_skip >= 16)
 		{
 		  start = NEXT_INSN (start);
-		  if (JUMP_P (start) || CALL_P (start))
+		  if ((JUMP_P (start) && asm_noperands (PATTERN (start)) < 0)
+		      || CALL_P (start))
 		    njumps--, isjump = 1;
 		  else
 		    isjump = 0;
@@ -38867,7 +38868,8 @@ ix86_avoid_jump_mispredicts (void)
       if (dump_file)
 	fprintf (dump_file, "Insn %i estimated to %i bytes\n",
 		 INSN_UID (insn), min_size);
-      if (JUMP_P (insn) || CALL_P (insn))
+      if ((JUMP_P (insn) && asm_noperands (PATTERN (insn)) < 0)
+	  || CALL_P (insn))
 	njumps++;
       else
 	continue;
@@ -38875,7 +38877,8 @@ ix86_avoid_jump_mispredicts (void)
       while (njumps > 3)
 	{
 	  start = NEXT_INSN (start);
-	  if (JUMP_P (start) || CALL_P (start))
+	  if ((JUMP_P (start) && asm_noperands (PATTERN (start)) < 0)
+	      || CALL_P (start))
 	    njumps--, isjump = 1;
 	  else
 	    isjump = 0;
--- gcc/testsuite/gcc.target/i386/pr59625.c.jj	2014-01-02 17:17:45.611680569 +0100
+++ gcc/testsuite/gcc.target/i386/pr59625.c	2014-01-02 17:17:37.000000000 +0100
@@ -0,0 +1,36 @@
+/* PR target/59625 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -mtune=atom" } */
+
+int
+foo (void)
+{
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  asm goto ("" : : : : lab);
+  return 0;
+lab:
+  return 1;
+}
+
+/* Verify we don't consider asm goto as a jump for four jumps limit
+   optimization.  asm goto doesn't have to contain a jump at all,
+   the branching to labels can happen through different means.  */
+/* { dg-final { scan-assembler-not "p2align\[^\n\r\]*\[\n\r]*\[^\n\r\]*p2align\[^\n\r\]*\[\n\r]*\[^\n\r\]*p2align\[^\n\r\]*\[\n\r]*\[^\n\r\]*p2align\[^\n\r\]*\[\n\r]*\[^\n\r\]*p2align\[^\n\r\]*\[\n\r]*\[^\n\r\]*p2align" } } */

	Jakub



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