[gcc r13-5962] ifcvt: Fix regression in aarch64/fcsel_1.c

[gcc r13-5962] ifcvt: Fix regression in aarch64/fcsel_1.c

[Richard Sandiford]

https://gcc.gnu.org/g:7d5a935070ed4a49e947a64b2391a1f5fa9b8f66

commit r13-5962-g7d5a935070ed4a49e947a64b2391a1f5fa9b8f66
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Mon Feb 13 11:38:45 2023 +0000

    ifcvt: Fix regression in aarch64/fcsel_1.c
    
    aarch64/fcsel_1.c contains:
    
    double
    f_2 (double a, double b, double c, double d)
    {
      if (a > b)
        return c;
      else
        return d;
    }
    
    which started failing in the GCC 12 timeframe.  When it passed,
    the RTL had the form:
    
    [A]
      (set (reg ret) (reg c))
      (set (pc) (if_then_else (gt ...) (label_ref ret) (pc)))
        edge to ret, fallthru to else
    else:
      (set (reg ret) (reg d))
        fallthru to ret
    ret:
      ...exit...
    
    i.e. a branch around.  Now the RTL has form:
    
    [B]
      (set (reg ret) (reg d))
      (set (pc) (if_then_else (gt ...) (label_ref then) (pc)))
        edge to then, fallthru to ret
    ret:
      ...exit...
    
    then:
      (set (reg ret) (reg c))
        edge to ret
    
    i.e. a branch out.
    
    Both are valid, of course, and there's no easy way to predict
    which we'll get.  But ifcvt canonicalises its representation on:
    
      if (cond) goto fallthru else goto non-fallthru
    
    That is, it canoncalises on the branch-around case for half-diamonds.
    It therefore wants to invert the comparison in [B] to get:
    
      if (...) goto ret else goto then
    
    But that isn't possible for strict FP gt, so the optimisation fails.
    
    Canonicalising on the branch-around case seems like the wrong choice for
    half diamonds.  The natural way of expressing a conditional branch is
    for the label_ref to be the "then" destination and pc to be the "else"
    destination.  And the natural choice of condition seems to be the one
    under which extra stuff *is* done, rather than the one under which extra
    stuff *isn't* done.  But that decision goes back at least 20 years and
    it doesn't seem like a good idea to change it in stage 4.
    
    This patch instead allows the internal structure to store the
    condition in inverted form.  For simplicity it handles only
    conditional moves, which is the one case that is needed
    to fix the known regression.  (There are probably unknown
    regressions too, but still.)
    
    gcc/
            * ifcvt.h (noce_if_info::cond_inverted): New field.
            * ifcvt.cc (cond_move_convert_if_block): Swap the then and else
            values when cond_inverted is true.
            (noce_find_if_block): Allow the condition to be inverted when
            handling conditional moves.

Diff:
---
 gcc/ifcvt.cc | 31 +++++++++++++++++++------------
 gcc/ifcvt.h  |  8 ++++++++
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
index 008796838f7..e4168d95ad9 100644
--- a/gcc/ifcvt.cc
+++ b/gcc/ifcvt.cc
@@ -4253,6 +4253,9 @@ cond_move_convert_if_block (struct noce_if_info *if_infop,
 	    e = dest;
 	}
 
+      if (if_infop->cond_inverted)
+	std::swap (t, e);
+
       target = noce_emit_cmove (if_infop, dest, code, cond_arg0, cond_arg1,
 				t, e);
       if (!target)
@@ -4405,7 +4408,6 @@ noce_find_if_block (basic_block test_bb, edge then_edge, edge else_edge,
   basic_block then_bb, else_bb, join_bb;
   bool then_else_reversed = false;
   rtx_insn *jump;
-  rtx cond;
   rtx_insn *cond_earliest;
   struct noce_if_info if_info;
   bool speed_p = optimize_bb_for_speed_p (test_bb);
@@ -4481,25 +4483,28 @@ noce_find_if_block (basic_block test_bb, edge then_edge, edge else_edge,
   if (! onlyjump_p (jump))
     return FALSE;
 
-  /* If this is not a standard conditional jump, we can't parse it.  */
-  cond = noce_get_condition (jump, &cond_earliest, then_else_reversed);
-  if (!cond)
-    return FALSE;
-
-  /* We must be comparing objects whose modes imply the size.  */
-  if (GET_MODE (XEXP (cond, 0)) == BLKmode)
-    return FALSE;
-
   /* Initialize an IF_INFO struct to pass around.  */
   memset (&if_info, 0, sizeof if_info);
   if_info.test_bb = test_bb;
   if_info.then_bb = then_bb;
   if_info.else_bb = else_bb;
   if_info.join_bb = join_bb;
-  if_info.cond = cond;
+  if_info.cond = noce_get_condition (jump, &cond_earliest,
+				     then_else_reversed);
   rtx_insn *rev_cond_earliest;
   if_info.rev_cond = noce_get_condition (jump, &rev_cond_earliest,
 					 !then_else_reversed);
+  if (!if_info.cond && !if_info.rev_cond)
+    return FALSE;
+  if (!if_info.cond)
+    {
+      std::swap (if_info.cond, if_info.rev_cond);
+      std::swap (cond_earliest, rev_cond_earliest);
+      if_info.cond_inverted = true;
+    }
+  /* We must be comparing objects whose modes imply the size.  */
+  if (GET_MODE (XEXP (if_info.cond, 0)) == BLKmode)
+    return FALSE;
   gcc_assert (if_info.rev_cond == NULL_RTX
 	      || rev_cond_earliest == cond_earliest);
   if_info.cond_earliest = cond_earliest;
@@ -4518,7 +4523,9 @@ noce_find_if_block (basic_block test_bb, edge then_edge, edge else_edge,
 
   /* Do the real work.  */
 
-  if (noce_process_if_block (&if_info))
+  /* ??? noce_process_if_block has not yet been updated to handle
+     inverted conditions.  */
+  if (!if_info.cond_inverted && noce_process_if_block (&if_info))
     return TRUE;
 
   if (HAVE_conditional_move
diff --git a/gcc/ifcvt.h b/gcc/ifcvt.h
index c6ea244aae1..be1385aabe4 100644
--- a/gcc/ifcvt.h
+++ b/gcc/ifcvt.h
@@ -86,6 +86,14 @@ struct noce_if_info
      form as well.  */
   bool then_else_reversed;
 
+  /* True if THEN_BB is conditional on !COND rather than COND.
+     This is used if:
+
+     - JUMP branches to THEN_BB on COND
+     - JUMP falls through to JOIN_BB on !COND
+     - COND cannot be reversed.  */
+  bool cond_inverted;
+
   /* True if the contents of then_bb and else_bb are a
      simple single set instruction.  */
   bool then_simple;