[PATCH] vect: Tighten vect_determine_precisions_from_range [PR113281]

Richard Sandiford richard.sandiford@arm.com
Sat Jan 27 15:43:25 GMT 2024


This was another PR caused by the way that
vect_determine_precisions_from_range handle shifts.  We tried to
narrow 32768 >> x to a 16-bit shift based on range information for
the inputs and outputs, with vect_recog_over_widening_pattern
(after PR110828) adjusting the shift amount.  But this doesn't
work for the case where x is in [16, 31], since then 32-bit
32768 >> x is a well-defined zero, whereas no well-defined
16-bit 32768 >> y will produce 0.

We could perhaps generate x < 16 ? 32768 >> x : 0 instead,
but since vect_determine_precisions_from_range was never really
supposed to rely on fix-ups, it seems better to fix that instead.

The patch also makes the code more selective about which codes
can be narrowed based on input and output ranges.  This showed
that vect_truncatable_operation_p was missing cases for
BIT_NOT_EXPR (equivalent to BIT_XOR_EXPR of -1) and NEGATE_EXPR
(equivalent to BIT_NOT_EXPR followed by a PLUS_EXPR of 1).

pr113281-1.c is the original testcase.  pr113281-[23].c failed
before the patch due to overly optimistic narrowing.  pr113281-[45].c
previously passed and are meant to protect against accidental
optimisation regressions.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Richard


gcc/
	PR target/113281
	* tree-vect-patterns.cc (vect_recog_over_widening_pattern): Remove
	workaround for right shifts.
	(vect_truncatable_operation_p): Handle NEGATE_EXPR and BIT_NOT_EXPR.
	(vect_determine_precisions_from_range): Be more selective about
	which codes can be narrowed based on their input and output ranges.
	For shifts, require at least one more bit of precision than the
	maximum shift amount.

gcc/testsuite/
	PR target/113281
	* gcc.dg/vect/pr113281-1.c: New test.
	* gcc.dg/vect/pr113281-2.c: Likewise.
	* gcc.dg/vect/pr113281-3.c: Likewise.
	* gcc.dg/vect/pr113281-4.c: Likewise.
	* gcc.dg/vect/pr113281-5.c: Likewise.
---
 gcc/testsuite/gcc.dg/vect/pr113281-1.c |  17 +++
 gcc/testsuite/gcc.dg/vect/pr113281-2.c |  50 +++++++++
 gcc/testsuite/gcc.dg/vect/pr113281-3.c |  39 +++++++
 gcc/testsuite/gcc.dg/vect/pr113281-4.c |  55 ++++++++++
 gcc/testsuite/gcc.dg/vect/pr113281-5.c |  66 ++++++++++++
 gcc/tree-vect-patterns.cc              | 144 +++++++++++++------------
 6 files changed, 305 insertions(+), 66 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113281-1.c
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113281-2.c
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113281-3.c
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113281-4.c
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr113281-5.c

diff --git a/gcc/testsuite/gcc.dg/vect/pr113281-1.c b/gcc/testsuite/gcc.dg/vect/pr113281-1.c
new file mode 100644
index 00000000000..6df4231cb5f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113281-1.c
@@ -0,0 +1,17 @@
+#include "tree-vect.h"
+
+unsigned char a;
+
+int main() {
+  check_vect ();
+
+  short b = a = 0;
+  for (; a != 19; a++)
+    if (a)
+      b = 32872 >> a;
+
+  if (b == 0)
+    return 0;
+  else
+    return 1;
+}
diff --git a/gcc/testsuite/gcc.dg/vect/pr113281-2.c b/gcc/testsuite/gcc.dg/vect/pr113281-2.c
new file mode 100644
index 00000000000..3a1170c28b6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113281-2.c
@@ -0,0 +1,50 @@
+/* { dg-do compile } */
+
+#define N 128
+
+short x[N];
+short y[N];
+
+void
+f1 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= y[i];
+}
+
+void
+f2 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 32 ? y[i] : 32);
+}
+
+void
+f3 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 31 ? y[i] : 31);
+}
+
+void
+f4 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] & 31);
+}
+
+void
+f5 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= 0x8000 >> y[i];
+}
+
+void
+f6 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= 0x8000 >> (y[i] & 31);
+}
+
+/* { dg-final { scan-tree-dump-not {can narrow[^\n]+>>} "vect" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/pr113281-3.c b/gcc/testsuite/gcc.dg/vect/pr113281-3.c
new file mode 100644
index 00000000000..5982dd2d16f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113281-3.c
@@ -0,0 +1,39 @@
+/* { dg-do compile } */
+
+#define N 128
+
+short x[N];
+short y[N];
+
+void
+f1 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 30 ? y[i] : 30);
+}
+
+void
+f2 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= ((y[i] & 15) + 2);
+}
+
+void
+f3 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 16 ? y[i] : 16);
+}
+
+void
+f4 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] = 32768 >> ((y[i] & 15) + 3);
+}
+
+/* { dg-final { scan-tree-dump {can narrow to signed:31 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to signed:18 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to signed:17 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to unsigned:19 without loss [^\n]+>>} "vect" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/pr113281-4.c b/gcc/testsuite/gcc.dg/vect/pr113281-4.c
new file mode 100644
index 00000000000..10fbc0e8405
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113281-4.c
@@ -0,0 +1,55 @@
+/* { dg-do compile } */
+
+#define N 128
+
+short x[N];
+short y[N];
+
+void
+f1 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] & 15);
+}
+
+void
+f2 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= ((y[i] & 7) + 8);
+}
+
+void
+f3 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= ((y[i] & 7) ^ 11);
+}
+
+void
+f4 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 15 ? y[i] : 15);
+}
+
+void
+f5 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] >>= (y[i] < 15 ? y[i] : 1);
+}
+
+void
+f6 (void)
+{
+  for (int i = 0; i < N; ++i)
+    x[i] = 32768 >> (y[i] & 15);
+}
+
+/* { dg-final { scan-tree-dump {:11:[^\n]+can narrow to signed:16 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {:18:[^\n]+can narrow to signed:16 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {:25:[^\n]+can narrow to signed:16 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {:32:[^\n]+can narrow to signed:16 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {:39:[^\n]+can narrow to signed:16 without loss [^\n]+>>} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to unsigned:16 without loss [^\n]+>>} "vect" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/pr113281-5.c b/gcc/testsuite/gcc.dg/vect/pr113281-5.c
new file mode 100644
index 00000000000..4a4571792e2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr113281-5.c
@@ -0,0 +1,66 @@
+/* { dg-do compile } */
+
+#define N 128
+
+short x[N];
+short y[N];
+
+void
+f1 (void)
+{
+  for (int i = 0; i < N; ++i)
+    {
+      int a = y[i];
+      int b = ~a;
+      x[i] = b;
+    }
+}
+
+void
+f2 (void)
+{
+  for (int i = 0; i < N; ++i)
+    {
+      int a = y[i];
+      int b = -a;
+      x[i] = b;
+    }
+}
+
+void
+f3 (void)
+{
+  for (int i = 0; i < N; ++i)
+    {
+      int a = x[i];
+      int b = a / y[i];
+      x[i] = b;
+    }
+}
+
+void
+f4 (void)
+{
+  for (int i = 0; i < N; ++i)
+    {
+      int a = x[i];
+      int b = a < y[i] ? a : y[i];
+      x[i] = b;
+    }
+}
+
+void
+f5 (void)
+{
+  for (int i = 0; i < N; ++i)
+    {
+      int a = x[i];
+      int b = a > y[i] ? a : y[i];
+      x[i] = b;
+    }
+}
+
+/* { dg-final { scan-tree-dump {can narrow to signed:17 without loss [^\n]+= -} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to signed:16 without loss [^\n]+= ~} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to signed:16 without loss [^\n]+ MIN_EXPR} "vect" } } */
+/* { dg-final { scan-tree-dump {can narrow to signed:16 without loss [^\n]+ MAX_EXPR} "vect" } } */
diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc
index f1a95ef34b9..d562f57920f 100644
--- a/gcc/tree-vect-patterns.cc
+++ b/gcc/tree-vect-patterns.cc
@@ -3164,46 +3164,9 @@ vect_recog_over_widening_pattern (vec_info *vinfo,
   tree ops[3] = {};
   for (unsigned int i = 1; i < first_op; ++i)
     ops[i - 1] = gimple_op (last_stmt, i);
-  /* For right shifts limit the shift operand.  */
   vect_convert_inputs (vinfo, last_stmt_info, nops, &ops[first_op - 1],
 		       op_type, &unprom[0], op_vectype);
 
-  /* Limit shift operands.  */
-  if (code == RSHIFT_EXPR)
-    {
-      wide_int min_value, max_value;
-      if (TREE_CODE (ops[1]) == INTEGER_CST)
-	ops[1] = wide_int_to_tree (op_type,
-				   wi::umin (wi::to_wide (ops[1]),
-					     new_precision - 1));
-      else if (!vect_get_range_info (ops[1], &min_value, &max_value)
-	       || wi::ge_p (max_value, new_precision, TYPE_SIGN (op_type)))
-	{
-	  /* ???  Note the following bad for SLP as that only supports
-	     same argument widened shifts and it un-CSEs same arguments.  */
-	  tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
-	  gimple *pattern_stmt
-	    = gimple_build_assign (new_var, MIN_EXPR, ops[1],
-				   build_int_cst (op_type, new_precision - 1));
-	  gimple_set_location (pattern_stmt, gimple_location (last_stmt));
-	  if (ops[1] == unprom[1].op && unprom[1].dt == vect_external_def)
-	    {
-	      if (edge e = vect_get_external_def_edge (vinfo, ops[1]))
-		{
-		  basic_block new_bb
-		    = gsi_insert_on_edge_immediate (e, pattern_stmt);
-		  gcc_assert (!new_bb);
-		}
-	      else
-		return NULL;
-	    }
-	  else
-	    append_pattern_def_seq (vinfo, last_stmt_info, pattern_stmt,
-				    op_vectype);
-	  ops[1] = new_var;
-	}
-    }
-
   /* Use the operation to produce a result of type OP_TYPE.  */
   tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
   gimple *pattern_stmt = gimple_build_assign (new_var, code,
@@ -6359,9 +6322,11 @@ vect_truncatable_operation_p (tree_code code)
 {
   switch (code)
     {
+    case NEGATE_EXPR:
     case PLUS_EXPR:
     case MINUS_EXPR:
     case MULT_EXPR:
+    case BIT_NOT_EXPR:
     case BIT_AND_EXPR:
     case BIT_IOR_EXPR:
     case BIT_XOR_EXPR:
@@ -6520,38 +6485,85 @@ vect_determine_precisions_from_range (stmt_vec_info stmt_info, gassign *stmt)
   unsigned int nops = gimple_num_ops (stmt);
 
   if (!vect_truncatable_operation_p (code))
-    /* Check that all relevant input operands are compatible, and update
-       [MIN_VALUE, MAX_VALUE] to include their ranges.  */
-    for (unsigned int i = 1; i < nops; ++i)
-      {
-	tree op = gimple_op (stmt, i);
-	if (TREE_CODE (op) == INTEGER_CST)
-	  {
-	    /* Don't require the integer to have RHS_TYPE (which it might
-	       not for things like shift amounts, etc.), but do require it
-	       to fit the type.  */
-	    if (!int_fits_type_p (op, type))
-	      return;
-
-	    min_value = wi::min (min_value, wi::to_wide (op, precision), sign);
-	    max_value = wi::max (max_value, wi::to_wide (op, precision), sign);
-	  }
-	else if (TREE_CODE (op) == SSA_NAME)
-	  {
-	    /* Ignore codes that don't take uniform arguments.  */
-	    if (!types_compatible_p (TREE_TYPE (op), type))
-	      return;
+    {
+      /* Handle operations that can be computed in type T if all inputs
+	 and outputs can be represented in type T.  Also handle left and
+	 right shifts, where (in addition) the maximum shift amount must
+	 be less than the number of bits in T.  */
+      bool is_shift;
+      switch (code)
+	{
+	case LSHIFT_EXPR:
+	case RSHIFT_EXPR:
+	  is_shift = true;
+	  break;
 
-	    wide_int op_min_value, op_max_value;
-	    if (!vect_get_range_info (op, &op_min_value, &op_max_value))
-	      return;
+	case ABS_EXPR:
+	case MIN_EXPR:
+	case MAX_EXPR:
+	case TRUNC_DIV_EXPR:
+	case CEIL_DIV_EXPR:
+	case FLOOR_DIV_EXPR:
+	case ROUND_DIV_EXPR:
+	case EXACT_DIV_EXPR:
+	  /* Modulus is excluded because it is typically calculated by doing
+	     a division, for which minimum signed / -1 isn't representable in
+	     the original signed type.  We could take the division range into
+	     account instead, if handling modulus ever becomes important.  */
+	  is_shift = false;
+	  break;
 
-	    min_value = wi::min (min_value, op_min_value, sign);
-	    max_value = wi::max (max_value, op_max_value, sign);
-	  }
-	else
+	default:
 	  return;
-      }
+	}
+      for (unsigned int i = 1; i < nops; ++i)
+	{
+	  tree op = gimple_op (stmt, i);
+	  wide_int op_min_value, op_max_value;
+	  if (TREE_CODE (op) == INTEGER_CST)
+	    {
+	      unsigned int op_precision = TYPE_PRECISION (TREE_TYPE (op));
+	      op_min_value = op_max_value = wi::to_wide (op, op_precision);
+	    }
+	  else if (TREE_CODE (op) == SSA_NAME)
+	    {
+	      if (!vect_get_range_info (op, &op_min_value, &op_max_value))
+		return;
+	    }
+	  else
+	    return;
+
+	  if (is_shift && i == 2)
+	    {
+	      /* There needs to be one more bit than the maximum shift amount.
+
+		 If the maximum shift amount is already 1 less than PRECISION
+		 then we can't narrow the shift further.  Dealing with that
+		 case first ensures that we can safely use an unsigned range
+		 below.
+
+		 op_min_value isn't relevant, since shifts by negative amounts
+		 are UB.  */
+	      if (wi::geu_p (op_max_value, precision - 1))
+		return;
+	      unsigned int min_bits = op_max_value.to_uhwi () + 1;
+
+	      /* As explained below, we can convert a signed shift into an
+		 unsigned shift if the sign bit is always clear.  At this
+		 point we've already processed the ranges of the output and
+		 the first input.  */
+	      auto op_sign = sign;
+	      if (sign == SIGNED && !wi::neg_p (min_value))
+		op_sign = UNSIGNED;
+	      op_min_value = wide_int::from (wi::min_value (min_bits, op_sign),
+					     precision, op_sign);
+	      op_max_value = wide_int::from (wi::max_value (min_bits, op_sign),
+					     precision, op_sign);
+	    }
+	  min_value = wi::min (min_value, op_min_value, sign);
+	  max_value = wi::max (max_value, op_max_value, sign);
+	}
+    }
 
   /* Try to switch signed types for unsigned types if we can.
      This is better for two reasons.  First, unsigned ops tend
-- 
2.25.1



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