Re: RFA: Fix tree-optimization/55524

[Joern Rennecke]

Quoting Richard Biener <richard.guent...@gmail.com>:

I don't see that.  It's merely a complication of optimal handling of
a * b +- c * d vs. just a * b +- c.  The pass does simple pattern matching
only, not doing a global optimal transform, so adding another special-case
is reasonable.  Special-casing just for single-use 2nd multiplication
simplifies the cases for example.

I have attached a version of the patch that uses this simpler test.
Currently bootstrapping / regtesting on i686-pc-linux-gnu .


gcc:
2013-04-09  Joern Rennecke <joern.renne...@embecosm.com>

        PR tree-optimization/55524
        * tree-ssa-math-opts.c
        (convert_mult_to_fma): Don't use an fms construct
        when we don't have an fms operation, but fnma, and it looks
        likely that we'll be able to use the latter.

gcc/testsuite:
2013-04-09  Joern Rennecke <joern.renne...@embecosm.com>

        PR tree-optimization/55524
        * gcc.target/epiphany/fnma-1.c: New test.

Index: testsuite/gcc.target/epiphany/fnma-1.c
===================================================================
--- testsuite/gcc.target/epiphany/fnma-1.c      (revision 0)
+++ testsuite/gcc.target/epiphany/fnma-1.c      (working copy)
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { scan-assembler-times "fmsub\[ \ta-zA-Z0-9\]*," 1 } } */
+
+float
+f (float ar, float ai, float br, float bi)
+{
+  return ar * br - ai * bi;
+}
Index: tree-ssa-math-opts.c
===================================================================
--- tree-ssa-math-opts.c        (revision 197578)
+++ tree-ssa-math-opts.c        (working copy)
@@ -2570,6 +2570,24 @@ convert_mult_to_fma (gimple mul_stmt, tr
          return false;
        }
 
+      /* If the subtrahend (gimple_assign_rhs2 (use_stmt)) is computed
+        by a MULT_EXPR that we'll visit later, we might be able to
+        get a more profitable match with fnma.
+        OTOH, if we don't, a negate / fma pair has likely lower latency
+        that a mult / subtract pair.  */
+      if (use_code == MINUS_EXPR && !negate_p
+         && gimple_assign_rhs1 (use_stmt) == result
+         && optab_handler (fms_optab, TYPE_MODE (type)) == CODE_FOR_nothing
+         && optab_handler (fnma_optab, TYPE_MODE (type)) != CODE_FOR_nothing)
+       {
+         tree rhs2 = gimple_assign_rhs2 (use_stmt);
+         gimple stmt2 = SSA_NAME_DEF_STMT (rhs2);
+
+         if (has_single_use (rhs2)
+             && gimple_assign_rhs_code (stmt2) == MULT_EXPR)
+           return false;
+       }
+
       /* We can't handle a * b + a * b.  */
       if (gimple_assign_rhs1 (use_stmt) == gimple_assign_rhs2 (use_stmt))
        return false;