[gcc r12-5369] middle-end: Fix FMA detection when inspecting gimple which have no LHS.

[Tamar Christina <tnfchris@gcc.gnu.org>]

https://gcc.gnu.org/g:1a0bce98dcf84dd0a7b3ac67b51ac68758cc7ad0

commit r12-5369-g1a0bce98dcf84dd0a7b3ac67b51ac68758cc7ad0
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Thu Nov 18 11:22:11 2021 +0000

    middle-end: Fix FMA detection when inspecting gimple which have no LHS.
    
    convert_mult_to_fma assumes that all gimple_assigns have a LHS set.  This
    assumption is however not true when an IFN is kept around just for the
    side-effects.  In those situations you have just the IFN and lhs will be null.
    
    Since there's no LHS, there also can't be any ADD and such it can't be an FMA
    so it's correct to just return early if no LHS.
    
    gcc/ChangeLog:
    
            PR tree-optimization/103253
            * tree-ssa-math-opts.c (convert_mult_to_fma): Check for LHS.
    
    gcc/testsuite/ChangeLog:
    
            PR tree-optimization/103253
            * gcc.dg/vect/pr103253.c: New test.

Diff:
---
 gcc/testsuite/gcc.dg/vect/pr103253.c | 16 ++++++++++++++++
 gcc/tree-ssa-math-opts.c             |  4 ++++
 2 files changed, 20 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/vect/pr103253.c b/gcc/testsuite/gcc.dg/vect/pr103253.c
new file mode 100644
index 00000000000..abe3f09f381
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr103253.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fopenmp } */
+/* { dg-additional-options "-O2 -fexceptions -fopenmp -fno-delete-dead-exceptions -fno-trapping-math" } */
+
+double
+do_work (double do_work_pri)
+{
+  int i;
+
+#pragma omp simd
+  for (i = 0; i < 17; ++i)
+    do_work_pri = (!i ? 0.5 : i) * 2.0;
+
+  return do_work_pri;
+}
+
diff --git a/gcc/tree-ssa-math-opts.c b/gcc/tree-ssa-math-opts.c
index c4a6492b50d..cc8496c3c32 100644
--- a/gcc/tree-ssa-math-opts.c
+++ b/gcc/tree-ssa-math-opts.c
@@ -3224,6 +3224,10 @@ convert_mult_to_fma (gimple *mul_stmt, tree op1, tree op2,
 		     fma_deferring_state *state, tree mul_cond = NULL_TREE)
 {
   tree mul_result = gimple_get_lhs (mul_stmt);
+  /* If there isn't a LHS then this can't be an FMA.  There can be no LHS
+     if the statement was left just for the side-effects.  */
+  if (!mul_result)
+    return false;
   tree type = TREE_TYPE (mul_result);
   gimple *use_stmt, *neguse_stmt;
   use_operand_p use_p;