ivbound: Introduce loop versioning pass for potentially overflowing IV's

Finds loops with potentially overflowing IV's and versions the loop with
a constant bound such that the bounded loop can have improved optimizations.

Signed-off-by: Michiel Derhaeg <michiel@synopsys.com>

Author: MichielDerhaeg

gcc/Makefile.in

@@ -1501,6 +1501,7 @@ OBJS = \
 	gimple-harden-control-flow.o \
 	gimple-laddress.o \
 	gimple-loop-interchange.o \
+	tree-ssa-loop-ivbound.o \
 	gimple-loop-jam.o \
 	gimple-loop-versioning.o \
 	gimple-low.o \

gcc/common.opt

@@ -3378,6 +3378,10 @@ fversion-loops-for-strides
 Common Var(flag_version_loops_for_strides) Optimization
 Version loops based on whether indices have a stride of one.
 
+fversion-loops-for-iv-overflow
+Common Var(flag_version_loops_for_iv_overflow) Optimization
+Version loops to guard against induction variable overflow.
+
 funwind-tables
 Common Var(flag_unwind_tables) Optimization
 Just generate unwind tables for exception handling.

gcc/passes.def

@@ -286,6 +286,10 @@ along with GCC; see the file COPYING3.  If not see
 	  NEXT_PASS (pass_loop_split);
 	  NEXT_PASS (pass_scev_cprop);
 	  NEXT_PASS (pass_loop_versioning);
+	  NEXT_PASS (pass_iv_bound_versioning);
+	  PUSH_INSERT_PASSES_WITHIN (pass_iv_bound_versioning)
+	      NEXT_PASS (pass_vrp, false);
+	  POP_INSERT_PASSES ()
 	  NEXT_PASS (pass_loop_jam);
 	  /* All unswitching, final value replacement and splitting can expose
 	     empty loops.  Remove them now.  */

gcc/testsuite/gcc.dg/ivbound-1.c

@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_unsigned_iv (unsigned int *arr, unsigned int n)
+{
+  for (unsigned int i = 0; i < n; i++)
+    arr[i] = i;
+}
+
+/* { dg-final { scan-tree-dump "candidate for versioning" "ivbound" } } */
+/* { dg-final { scan-tree-dump "Successfully versioned loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-10.c

@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_unsigned_char_iv (unsigned char *arr, unsigned char n)
+{
+  for (unsigned char i = 0; i < n; i++)
+    arr[i] = i;
+}
+
+/* { dg-final { scan-tree-dump "candidate for versioning" "ivbound" } } */
+/* { dg-final { scan-tree-dump "Successfully versioned loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-11.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_decrementing_iv (unsigned int *arr, unsigned int n)
+{
+  for (unsigned int i = n; i > 0; i--)
+    arr[i - 1] = i;
+}
+
+/* { dg-final { scan-tree-dump-not "Successfully versioned loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-12.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_pointer_iv (int *start, int *end)
+{
+  for (int *p = start; p < end; p++)
+    *p = 0;
+}
+
+/* { dg-final { scan-tree-dump-not "Found unsigned IV in loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-13.c

@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_nested_different_bounds (unsigned int *arr, unsigned int n, unsigned int m)
+{
+  for (unsigned int i = 0; i < n; i++)
+    for (unsigned int j = 0; j < m; j++)
+      arr[i * m + j] = i + j;
+}
+
+/* Both bounds (n, m) are invariant to both loops, so we can hoist.  */
+/* { dg-final { scan-tree-dump "Hoisting versioning from loop" "ivbound" } } */
+/* { dg-final { scan-tree-dump "Successfully versioned loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-14.c

@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_inner_bound_not_invariant (unsigned int *arr, unsigned int n)
+{
+  for (unsigned int i = 0; i < n; i++)
+    for (unsigned int j = 0; j < i; j++)  /* bound 'i' varies with outer loop */
+      arr[i * n + j] = i + j;
+}
+
+/* Inner loop's bound is not invariant to outer, cannot hoist to outer.  */
+/* { dg-final { scan-tree-dump "Successfully versioned loop" "ivbound" } } */

gcc/testsuite/gcc.dg/ivbound-15.c

@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fversion-loops-for-iv-overflow -fdump-tree-ivbound-details" } */
+
+void
+test_outer_signed_inner_unsigned (int *arr, int n, unsigned int m)
+{
+  for (int i = 0; i < n; i++)  /* signed - no versioning needed */
+    for (unsigned int j = 0; j < m; j++)  /* unsigned - needs versioning */
+      arr[i * m + j] = i + j;
+}
+
+/* Inner loop needs versioning.  Bound 'm' is invariant to outer, so hoist.  */
+/* { dg-final { scan-tree-dump "Successfully versioned loop" "ivbound" } } */
+/* { dg-final { scan-tree-dump "Hoisting versioning from loop" "ivbound" } } */