From 70a62009181f66d1d1c90d3c74de38e153c96eb0 Mon Sep 17 00:00:00 2001
From: Ilya Leoshkevich <iii@linux.ibm.com>
Date: Wed, 3 Jun 2020 20:55:20 +0200
Subject: [PATCH] tree-ssa-threadbackward.c (profitable_jump_thread_path): Do
 not allow __builtin_constant_p.

Linux Kernel (specifically, drivers/leds/trigger/ledtrig-cpu.c) build
with GCC 10 fails on s390 with "impossible constraint".

Explanation by Jeff Law:

```
So what we have is a b_c_p at the start of an if-else chain.  Subsequent
tests on the "true" arm of the the b_c_p test may throw us off the
constant path (because the constants are out of range).  Once all the
tests are passed (it's constant and the constant is in range) the true
arm's terminal block has a special asm that requires a constant
argument.   In the case where we get to the terminal block on the true
arm, the argument to the b_c_p is used as the constant argument to the
special asm.

At first glace jump threading seems to be doing the right thing.  Except
that we end up with two paths to that terminal block with the special
asm, one for each of the two constant arguments to the b_c_p call.
Naturally since that same value is used in the asm, we have to introduce
a PHI to select between them at the head of the terminal block.   Now
the argument in the asm is no longer constant and boom we fail.
```

Fix by disallowing __builtin_constant_p on threading paths.

gcc/ChangeLog:

2020-06-03  Ilya Leoshkevich  <iii@linux.ibm.com>

	* tree-ssa-threadbackward.c (thread_jumps::profitable_jump_thread_path):
	Do not allow __builtin_constant_p on a threading path.

gcc/testsuite/ChangeLog:

2020-06-03  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/builtin-constant-p-threading.c: New test.
---
 .../s390/builtin-constant-p-threading.c       | 46 +++++++++++++++++++
 gcc/tree-ssa-threadbackward.c                 |  7 ++-
 2 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c

diff --git a/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c b/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c
new file mode 100644
index 00000000000..5f0acdce0b0
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=z196 -mzarch" } */
+
+typedef struct
+{
+  int counter;
+} atomic_t;
+
+static inline __attribute__ ((__gnu_inline__)) int
+__atomic_add (int val, int *ptr)
+{
+  int old;
+  asm volatile("laa %[old],%[val],%[ptr]\n"
+	       : [old] "=d" (old), [ptr] "+Q"(*ptr)
+	       : [val] "d" (val)
+	       : "cc", "memory");
+  return old;
+}
+
+static inline __attribute__ ((__gnu_inline__)) void
+__atomic_add_const (int val, int *ptr)
+{
+  asm volatile("asi %[ptr],%[val]\n"
+	       : [ptr] "+Q" (*ptr)
+	       : [val] "i" (val)
+	       : "cc", "memory");
+}
+
+static inline __attribute__ ((__gnu_inline__)) void
+atomic_add (int i, atomic_t *v)
+{
+  if (__builtin_constant_p (i) && (i > -129) && (i < 128))
+    {
+      __atomic_add_const (i, &v->counter);
+      return;
+    }
+  __atomic_add (i, &v->counter);
+}
+
+static atomic_t num_active_cpus = { (0) };
+
+void
+ledtrig_cpu (_Bool is_active)
+{
+  atomic_add (is_active ? 1 : -1, &num_active_cpus);
+}
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 327628f1662..30f692672d9 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -259,8 +259,13 @@ thread_jumps::profitable_jump_thread_path (basic_block bbi, tree name,
 	       !gsi_end_p (gsi);
 	       gsi_next_nondebug (&gsi))
 	    {
+	      /* Do not allow OpenACC loop markers and __builtin_constant_p on
+		 threading paths.  The latter is disallowed, because an
+		 expression might be constant on two threading paths, and
+		 become non-constant (i.e.: phi) when they merge.  */
 	      gimple *stmt = gsi_stmt (gsi);
-	      if (gimple_call_internal_p (stmt, IFN_UNIQUE))
+	      if (gimple_call_internal_p (stmt, IFN_UNIQUE)
+		  || gimple_call_builtin_p (stmt, BUILT_IN_CONSTANT_P))
 		{
 		  m_path.pop ();
 		  return NULL;