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

Tamar Christina tamar.christina@arm.com
Tue Nov 16 15:35:14 GMT 2021


Hi All,

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.

Bootstrapped Regtested on aarch64-none-linux-gnu,
x86_64-pc-linux-gnu and no regressions.

Ok for master?

Thanks,
Tamar



gcc/ChangeLog:

	PR tree-optimizations/103253
	* tree-ssa-math-opts.c (convert_mult_to_fma): Check for LHS.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/pr103253.c: New test.

--- inline copy of patch -- 
diff --git a/gcc/testsuite/gcc.dg/vect/pr103253.c b/gcc/testsuite/gcc.dg/vect/pr103253.c
new file mode 100644
index 0000000000000000000000000000000000000000..abe3f09f3818d79a53f2aa962c6b6c06855d618e
--- /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 c4a6492b50df25b4cf296a75bd51e5af34eeacc7..cc8496c3c325f3cc303a90b9b9cac383e5a7942d 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;


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