[PATCH] avoid-store-forwarding: Fix base register initialization when eliminating loads [PR117835]

[Konstantinos Eleftheriou]

From: kelefth <konstantinos.elefther...@vrull.eu>

During the initialization of the base register for the zero-offset store, in
the case that we are eliminating the load, we used a paradoxical subreg
assuming that we don't care about the higher bits of the register. This led to
writing wrong values when we were not updating the whole register.

This patch fixes the issue by zero-extending the value stored in the base
register instead of using a paradoxical subreg.

Bootstrapped/regtested on x86 and AArch64.

        PR rtl-optimization/117835

gcc/ChangeLog:

        * avoid-store-forwarding.cc 
(store_forwarding_analyzer::process_store_forwarding):
        Zero-extend the value stored in the base register instead of using a
        paradoxical subreg.

gcc/testsuite/ChangeLog:

        * gcc.target/i386/pr117835.c: New test.
---
 gcc/avoid-store-forwarding.cc            |  6 +-----
 gcc/testsuite/gcc.target/i386/pr117835.c | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/i386/pr117835.c

diff --git a/gcc/avoid-store-forwarding.cc b/gcc/avoid-store-forwarding.cc
index 1b8c35bc6cb..fa83e10fedc 100644
--- a/gcc/avoid-store-forwarding.cc
+++ b/gcc/avoid-store-forwarding.cc
@@ -238,11 +238,7 @@ process_store_forwarding (vec<store_fwd_info> &stores, 
rtx_insn *load_insn,
        {
          start_sequence ();
 
-         /* We can use a paradoxical subreg to force this to a wider mode, as
-            the only use will be inserting the bits (i.e., we don't care about
-            the value of the higher bits).  */
-         rtx ext0 = lowpart_subreg (GET_MODE (dest), it->mov_reg,
-                                    GET_MODE (it->mov_reg));
+         rtx ext0 = gen_rtx_ZERO_EXTEND (GET_MODE (dest), it->mov_reg);
          if (ext0)
            {
              rtx_insn *move0 = emit_move_insn (dest, ext0);
diff --git a/gcc/testsuite/gcc.target/i386/pr117835.c 
b/gcc/testsuite/gcc.target/i386/pr117835.c
new file mode 100644
index 00000000000..eac71aac916
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr117835.c
@@ -0,0 +1,20 @@
+/* { dg-do run } */
+/* { dg-options "-O -favoid-store-forwarding -mno-push-args 
--param=store-forwarding-max-distance=0 -Wno-psabi" } */
+
+typedef __attribute__((__vector_size__ (64))) unsigned short V;
+
+__attribute__((__noipa__)) V
+foo (V v, V)
+{
+  return v;
+}
+
+int main ()
+{
+  V a = (V){3, 5, 0, 8, 9, 3, 5, 1, 3, 4, 2, 5, 5, 0, 5, 3, 61886};
+  V b = (V){6, 80, 15, 2, 2, 1, 1, 3, 5};
+  V x = foo (a, b);
+  for (unsigned i = 0; i < sizeof(x)/sizeof(x[0]); i++)
+    if (x[i] != a[i])
+      __builtin_abort();
+}
\ No newline at end of file
-- 
2.47.0