middle-end: Fix logical shift truncation (PR rtl-optimization/91838)

This fixes a fall-out from a patch I had submitted two years ago which started
allowing simplify-rtx to fold logical right shifts by offsets a followed by b
into >> (a + b).

However this can generate inefficient code when the resulting shift count ends
up being the same as the size of the shift mode.  This will create some
undefined behavior on most platforms.

This patch changes to code to truncate to 0 if the shift amount goes out of
range.  Before my older patch this used to happen in combine when it saw the
two shifts.  However since we combine them here combine never gets a chance to
truncate them.

The issue mostly affects GCC 8 and 9 since on 10 the back-end knows how to deal
with this shift constant but it's better to do the right thing in simplify-rtx.

Note that this doesn't take care of the Arithmetic shift where you could replace
the constant with MODE_BITS (mode) - 1, but that's not a regression so punting it.

gcc/ChangeLog:

	PR rtl-optimization/91838
	* simplify-rtx.c (simplify_binary_operation_1): Update LSHIFTRT case
	to truncate if allowed or reject combination.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/91838
	* g++.dg/pr91838.C: New test.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index eb5105f0b19b387ea66795d272325cafcd220f7c..f44a09d782231d920faac91e9d290060849c98ed 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2020-01-31  Tamar Christina  <tamar.christina@arm.com>
+
+       PR rtl-optimization/91838
+       * simplify-rtx.c (simplify_binary_operation_1): Update LSHIFTRT case
+       to truncate if allowed or reject combination.
+
 2020-01-31  Andrew Stubbs  <ams@codesourcery.com>
 
        * tree-ssa-loop-ivopts.c (get_iv): Use sizetype for zero-step.

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index eff1d07a2533c7bda5f0529cd318f08e6d5209d6..dd3d85156c3ed2bcde1b2926b317b29759944ddd 100644 (file)
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -3647,9 +3647,21 @@ simplify_binary_operation_1 (enum rtx_code code, machine_mode mode,
        {
          rtx tmp = gen_int_shift_amount
            (inner_mode, INTVAL (XEXP (SUBREG_REG (op0), 1)) + INTVAL (op1));
-         tmp = simplify_gen_binary (code, inner_mode,
-                                    XEXP (SUBREG_REG (op0), 0),
-                                    tmp);
+
+        /* Combine would usually zero out the value when combining two
+           local shifts and the range becomes larger or equal to the mode.
+           However since we fold away one of the shifts here combine won't
+           see it so we should immediately zero the result if it's out of
+           range.  */
+        if (code == LSHIFTRT
+            && INTVAL (tmp) >= GET_MODE_BITSIZE (inner_mode))
+         tmp = const0_rtx;
+        else
+          tmp = simplify_gen_binary (code,
+                                     inner_mode,
+                                     XEXP (SUBREG_REG (op0), 0),
+                                     tmp);
+
          return lowpart_subreg (int_mode, tmp, inner_mode);
        }
 

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index e4e612ada921d2174d9dbaa938735d2472ceddcd..8b1dcf238552e8a63d2ed70f1e69ee41796f2e3e 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-01-31  Tamar Christina  <tamar.christina@arm.com>
+
+       PR rtl-optimization/91838
+       * g++.dg/pr91838.C: New test.
+
 2020-01-30  David Malcolm  <dmalcolm@redhat.com>
 
        * gcc.dg/analyzer/malloc-1.c: Remove include of <string.h>.

diff --git a/gcc/testsuite/g++.dg/pr91838.C b/gcc/testsuite/g++.dg/pr91838.C
new file mode 100644 (file)
index 0000000..4dbaef0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr91838.C
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O2" } */
+/* { dg-skip-if "" { *-*-* } {-std=c++98} } */
+
+using T = unsigned char; // or ushort, or uint
+using V [[gnu::vector_size(8)]] = T;
+V f(V x) {
+  return x >> 8 * sizeof(T);
+}
+
+/* { dg-final { scan-assembler {pxor\s+%xmm0,\s+%xmm0} { target x86_64-*-* } } } */