[PR target/118248] Avoid bogus alloca call in RISC-V backend

This is Jakub's patch and Ian's testcase for the slightly vexing fault building
the D runtime with an s390x-x-riscv cross compiler.

The core issue is we're allocating a vector to hold temporary registers
unconditionally, including cases where the vector isn't needed because the loop
isn't going to iterate.

In the cases where the vector isn't needed the length is computed with an
expression (x / y) - 1 where x / y will be zero.  The alloca(-1) on the s390
platform triggers a fault.  We haven't seen the fault with an x86 cross, but we
can certainly see the bogus value being passed to alloca with a debugger.

Jakub patch just conditionalizes the whole block in a sensible way.  So it
looks larger than it really is.  I thought it might be better to do a bit of
manual CSE on this code to make it even more obvious, but I think we're
ultimately OK here.

Ian provided the testcase, collapsed down into equivalent C code. Again, it
doesn't fault on an x86-x-riscv, but I can see the incorrect behavior with a
debugger.

And a shout-out to Stefan for providing a docker based reproducer, it really
helped track this down.

	PR target/118248
gcc/
	* config/riscv/riscv-string.cc (riscv_block_move_straight): Only
	allocate REGS buffer if it will be needed.

gcc/testsuite
	* gcc.target/riscv/pr118248.c: New test.

diff --git a/gcc/config/riscv/riscv-string.cc b/gcc/config/riscv/riscv-string.cc
index 408eb07e87f38776d332e0718718aa78b906a1eb..90801899ec8e3cabc4ab4670ec1bab8e336c8024 100644 (file)
--- a/gcc/config/riscv/riscv-string.cc
+++ b/gcc/config/riscv/riscv-string.cc
@@ -804,7 +804,7 @@ static void
 riscv_block_move_straight (rtx dest, rtx src, unsigned HOST_WIDE_INT length,
                           unsigned HOST_WIDE_INT align)
 {
-  unsigned HOST_WIDE_INT offset, delta;
+  unsigned HOST_WIDE_INT offset = 0, delta;
   unsigned HOST_WIDE_INT bits;
   int i;
   enum machine_mode mode;
@@ -815,20 +815,25 @@ riscv_block_move_straight (rtx dest, rtx src, unsigned HOST_WIDE_INT length,
   mode = mode_for_size (bits, MODE_INT, 0).require ();
   delta = bits / BITS_PER_UNIT;
 
-  /* Allocate a buffer for the temporary registers.  */
-  regs = XALLOCAVEC (rtx, length / delta - 1);
-
-  /* Load as many BITS-sized chunks as possible.  Use a normal load if
-     the source has enough alignment, otherwise use left/right pairs.  */
-  for (offset = 0, i = 0; offset + 2 * delta <= length; offset += delta, i++)
+  if (2 * delta <= length)
     {
-      regs[i] = gen_reg_rtx (mode);
-      riscv_emit_move (regs[i], adjust_address (src, mode, offset));
-    }
+      /* Allocate a buffer for the temporary registers.  */
+      regs = XALLOCAVEC (rtx, length / delta - 1);
+
+      /* Load as many BITS-sized chunks as possible.  Use a normal load if
+        the source has enough alignment, otherwise use left/right pairs.  */
+      for (offset = 0, i = 0; offset + 2 * delta <= length;
+          offset += delta, i++)
+       {
+         regs[i] = gen_reg_rtx (mode);
+         riscv_emit_move (regs[i], adjust_address (src, mode, offset));
+       }
 
-  /* Copy the chunks to the destination.  */
-  for (offset = 0, i = 0; offset + 2 * delta <= length; offset += delta, i++)
-    riscv_emit_move (adjust_address (dest, mode, offset), regs[i]);
+      /* Copy the chunks to the destination.  */
+      for (offset = 0, i = 0; offset + 2 * delta <= length;
+          offset += delta, i++)
+       riscv_emit_move (adjust_address (dest, mode, offset), regs[i]);
+    }
 
   /* Mop up any left-over bytes.  */
   if (offset < length)

diff --git a/gcc/testsuite/gcc.target/riscv/pr118248.c b/gcc/testsuite/gcc.target/riscv/pr118248.c
new file mode 100644 (file)
index 0000000..05bf002
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr118248.c
@@ -0,0 +1,26 @@
+/* This does not fault with an x86 cross to risc-v, but does
+   with s390 cross to risc-v, probably due to rounding of the
+   argument to the alloca call within the risc-v backend.  */
+/* { dg-do compile } */
+struct char100
+{
+    char data[100];
+};
+
+struct s118248
+{
+    void **vtbl;
+    struct char100 data;
+};
+
+void sink(struct char100 *buf);
+
+struct s118248 *pr118248(struct s118248 *pthis)
+{
+    struct char100 buf;
+    sink(&buf);
+    pthis->data = buf;
+    return pthis;
+}
+
+