[PATCH, rs6000] Do not do gimple-folding of expressions that are missing their LHS

[Will Schmidt <will_schmidt at vnet dot ibm dot com>]

Hi,

   
Do not do the gimple-folding optimizations of expressions that are
missing their LHS.  (Preventing an ICE on invalid code).
    
This was noticed during debug of PR81317, but is not a fix for that PR.
This is based on a patch suggested by Segher.

(This will need a revisit if/when we get as far as doing early gimple
folding for expressions without a lhs, but for now, this seems sufficient).

This seems straightforward.  regtest going on ppc64LE just in case.
OK for trunk?
    
[gcc]

	2017-07-12  Will Schmidt  <will_schmidt@vnet.ibm.com>

	* config/rs6000/rs6000.c (rs6000_gimple_fold_builtin): Return
	early if there is no lhs.

[gcc/testsuite]

	2017-07-12  Will Schmidt  <will_schmidt@vnet.ibm.com>

	* gcc.target/powerpc/fold-vec-missing-lhs.c: New.

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 10c5521..e21b56f 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -16297,6 +16297,9 @@ rs6000_gimple_fold_builtin (gimple_stmt_iterator *gsi)
     = (enum rs6000_builtins) DECL_FUNCTION_CODE (fndecl);
   tree arg0, arg1, lhs;
 
+  /*  Generic solution to prevent gimple folding of code without a LHS.  */
+  if (!gimple_call_lhs (stmt)) return false;
+
   switch (fn_code)
     {
     /* Flavors of vec_add.  We deliberately don't expand
diff --git a/gcc/testsuite/gcc.target/powerpc/fold-vec-missing-lhs.c b/gcc/testsuite/gcc.target/powerpc/fold-vec-missing-lhs.c
new file mode 100644
index 0000000..d62f913
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/fold-vec-missing-lhs.c
@@ -0,0 +1,24 @@
+/* This test is meant to verify that the gimple-folding does not
+occur when the LHS portion of an expression is missing.
+Though we would consider this invalid code, this should not generate an ICE.
+This was noticed during debug of PR81317.  */
+
+/* { dg-do compile } */
+/* { dg-require-effective-target powerpc_altivec_ok } */
+/* { dg-options "-maltivec" } */
+
+#include <altivec.h>
+
+vector signed short
+test1_nolhs (vector bool short x, vector signed short y)
+{
+  vec_add (x, y);
+  return vec_add (x, y);
+}
+
+vector signed short
+test2_nolhs (vector signed short x, vector bool short y)
+{
+  vec_add (x, y);
+  return vec_add (x, y);
+}