[PATCH] Allow FSM to thread single block cases too

[Jeff Law <law at redhat dot com>]

One of the cases that was missing in the FSM support is threading when 
the path is a single block.  ie, a control statement's output can be 
statically determined just by looking at PHIs in the control statement's 
block for one or incoming edges.

This is necessary to fix a regression if I turn off the old jump 
threader's backedge support.  Just as important, Jan has in the past 
asked about a trivial jump threader to be run during early 
optimizations.  Limiting the FSM bits to this case would likely satisfy 
that need in the future.

Bootstrapped and regression tested on x86_64-linux-gnu.  Installed on 
the trunk.

Jeff

commit a53bb29a1dffd329aa6235b88b0c2a830aa5a59e
Author: Jeff Law <law@redhat.com>
Date:   Tue Oct 13 06:19:20 2015 -0600

    [PATCH] Allow FSM to thread single block cases too
    
    	* tree-ssa-threadbackward.c (fsm_find_control_statement_thread_paths):
    	Allow single block jump threading paths.
    
    	* gcc.dg/tree-ssa/ssa-thread-13.c: New test.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index d71bcd2..caab533 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2015-10-13  Jeff Law  <law@redhat.com>
+
+	* tree-ssa-threadbackward.c (fsm_find_control_statement_thread_paths):
+	Allow single block jump threading paths.
+
 2015-10-13  Tom de Vries  <tom@codesourcery.com>
 
 	PR tree-optimization/67476
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 4a08f0f..acf6df5 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2015-10-13  Jeff Law  <law@redhat.com>
+
+	* gcc.dg/tree-ssa/ssa-thread-13.c: New test.
+
 2015-10-12  Jeff Law  <law@redhat.com>
 
 	* gcc.dg/tree-ssa/ssa-thread-12.c: New test.
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-13.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-13.c
new file mode 100644
index 0000000..5051d11
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-13.c
@@ -0,0 +1,70 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-vrp1-details" } */
+/* { dg-final { scan-tree-dump "FSM" "vrp1" } } */
+
+typedef struct rtx_def *rtx;
+typedef const struct rtx_def *const_rtx;
+enum rtx_code
+{
+  UNKNOWN, VALUE, DEBUG_EXPR, EXPR_LIST, INSN_LIST, SEQUENCE, ADDRESS,
+    DEBUG_INSN, INSN, JUMP_INSN, CALL_INSN, BARRIER, CODE_LABEL, NOTE,
+    COND_EXEC, PARALLEL, ASM_INPUT, ASM_OPERANDS, UNSPEC, UNSPEC_VOLATILE,
+    ADDR_VEC, ADDR_DIFF_VEC, PREFETCH, SET, USE, CLOBBER, CALL, RETURN,
+    EH_RETURN, TRAP_IF, CONST_INT, CONST_FIXED, CONST_DOUBLE, CONST_VECTOR,
+    CONST_STRING, CONST, PC, REG, SCRATCH, SUBREG, STRICT_LOW_PART, CONCAT,
+    CONCATN, MEM, LABEL_REF, SYMBOL_REF, CC0, IF_THEN_ELSE, COMPARE, PLUS,
+    MINUS, NEG, MULT, SS_MULT, US_MULT, DIV, SS_DIV, US_DIV, MOD, UDIV, UMOD,
+    AND, IOR, XOR, NOT, ASHIFT, ROTATE, ASHIFTRT, LSHIFTRT, ROTATERT, SMIN,
+    SMAX, UMIN, UMAX, PRE_DEC, PRE_INC, POST_DEC, POST_INC, PRE_MODIFY,
+    POST_MODIFY, NE, EQ, GE, GT, LE, LT, GEU, GTU, LEU, LTU, UNORDERED,
+    ORDERED, UNEQ, UNGE, UNGT, UNLE, UNLT, LTGT, SIGN_EXTEND, ZERO_EXTEND,
+    TRUNCATE, FLOAT_EXTEND, FLOAT_TRUNCATE, FLOAT, FIX, UNSIGNED_FLOAT,
+    UNSIGNED_FIX, FRACT_CONVERT, UNSIGNED_FRACT_CONVERT, SAT_FRACT,
+    UNSIGNED_SAT_FRACT, ABS, SQRT, BSWAP, FFS, CLZ, CTZ, POPCOUNT, PARITY,
+    SIGN_EXTRACT, ZERO_EXTRACT, HIGH, LO_SUM, VEC_MERGE, VEC_SELECT,
+    VEC_CONCAT, VEC_DUPLICATE, SS_PLUS, US_PLUS, SS_MINUS, SS_NEG, US_NEG,
+    SS_ABS, SS_ASHIFT, US_ASHIFT, US_MINUS, SS_TRUNCATE, US_TRUNCATE, FMA,
+    VAR_LOCATION, DEBUG_IMPLICIT_PTR, ENTRY_VALUE, LAST_AND_UNUSED_RTX_CODE
+};
+union rtunion_def
+{
+  rtx rt_rtx;
+};
+typedef union rtunion_def rtunion;
+struct rtx_def
+{
+  __extension__ enum rtx_code code:16;
+  union u
+  {
+    rtunion fld[1];
+  }
+  u;
+};
+
+unsigned int rtx_cost (rtx, enum rtx_code, unsigned char);
+rtx single_set_2 (const_rtx, rtx);
+
+unsigned
+seq_cost (const_rtx seq, unsigned char speed)
+{
+  unsigned cost = 0;
+  rtx set;
+  for (; seq; seq = (((seq)->u.fld[2]).rt_rtx))
+    {
+      set =
+	(((((enum rtx_code) (seq)->code) == INSN)
+	  || (((enum rtx_code) (seq)->code) == DEBUG_INSN)
+	  || (((enum rtx_code) (seq)->code) == JUMP_INSN)
+	  || (((enum rtx_code) (seq)->code) ==
+	      CALL_INSN)) ? (((enum rtx_code) ((((seq)->u.fld[4]).rt_rtx))->
+			      code) ==
+			     SET ? (((seq)->u.fld[4]).
+				    rt_rtx) : single_set_2 (seq,
+							    (((seq)->u.
+							      fld[4]).
+							     rt_rtx))) : (rtx)
+	 0);
+      if (set)
+	cost += rtx_cost (set, SET, speed);
+    }
+}
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 5be6ee4..9128094 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -211,10 +211,6 @@ fsm_find_control_statement_thread_paths (tree name,
 	continue;
 
       int path_length = path->length ();
-      /* A path with less than 2 basic blocks should not be jump-threaded.  */
-      if (path_length < 2)
-	continue;
-
       if (path_length > PARAM_VALUE (PARAM_MAX_FSM_THREAD_LENGTH))
 	{
 	  if (dump_file && (dump_flags & TDF_DETAILS))