[gcc r11-8213] rtlanal: Don't assume that calls write to a global SP [PR99596]

[gcc r11-8213] rtlanal: Don't assume that calls write to a global SP [PR99596]

[Richard Sandiford]

https://gcc.gnu.org/g:b4d6af55fe55c0eab87ab875bfd0346677e12236

commit r11-8213-gb4d6af55fe55c0eab87ab875bfd0346677e12236
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Fri Apr 16 12:38:01 2021 +0100

    rtlanal: Don't assume that calls write to a global SP [PR99596]
    
    This patch is a GCC 11 regression caused by the rtl-ssa code.
    Normally we treat calls as containing a potential set of a global
    register, but DF makes a sensible exception for the stack pointer:
    
          if (i == STACK_POINTER_REGNUM)
            /* The stack ptr is used (honorarily) by a CALL insn.  */
            df_ref_record (DF_REF_BASE, collection_rec, regno_reg_rtx[i],
                           NULL, bb, insn_info, DF_REF_REG_USE,
                           DF_REF_CALL_STACK_USAGE | flags);
          else if (global_regs[i])
            {
              /* Calls to const functions cannot access any global registers and
                 calls to pure functions cannot set them.  All other calls may
                 reference any of the global registers, so they are recorded as
                 used. */
    
    The only DF definition of SP was therefore the one in the entry block.
    However, the rtlanal.c rtx_properties code (wrongly) assumed that calls
    also clobbered the global SP.  This led to multiple definitions of SP
    when we only expected one.
    
    This patch tightens the rtlanal.c handling of global registers
    to match the DF approach.
    
    gcc/
            PR rtl-optimization/99596
            * rtlanal.c (rtx_properties::try_to_add_insn): Don't add global
            register accesses for const calls.  Assume that pure functions
            can only read from global registers.  Ignore cases in which
            the stack pointer has been marked global.
    
    gcc/testsuite/
            PR rtl-optimization/99596
            * gcc.target/arm/pr99596.c: New test.

Diff:
---
 gcc/rtlanal.c                          | 20 +++++++++++++++-----
 gcc/testsuite/gcc.target/arm/pr99596.c | 18 ++++++++++++++++++
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c
index c35386bccbd..170420a610b 100644
--- a/gcc/rtlanal.c
+++ b/gcc/rtlanal.c
@@ -2311,15 +2311,25 @@ rtx_properties::try_to_add_insn (const rtx_insn *insn, bool include_notes)
 {
   if (CALL_P (insn))
     {
-      /* Adding the global registers first removes a situation in which
+      /* Non-const functions can read from global registers.  Impure
+	 functions can also set them.
+
+	 Adding the global registers first removes a situation in which
 	 a fixed-form clobber of register R could come before a real set
 	 of register R.  */
-      if (!hard_reg_set_empty_p (global_reg_set))
+      if (!hard_reg_set_empty_p (global_reg_set)
+	  && !RTL_CONST_CALL_P (insn))
 	{
-	  unsigned int flags = (rtx_obj_flags::IS_READ
-				| rtx_obj_flags::IS_WRITE);
+	  unsigned int flags = rtx_obj_flags::IS_READ;
+	  if (!RTL_PURE_CALL_P (insn))
+	    flags |= rtx_obj_flags::IS_WRITE;
 	  for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; ++regno)
-	    if (global_regs[regno] && ref_iter != ref_end)
+	    /* As a special case, the stack pointer is invariant across calls
+	       even if it has been marked global; see the corresponding
+	       handling in df_get_call_refs.  */
+	    if (regno != STACK_POINTER_REGNUM
+		&& global_regs[regno]
+		&& ref_iter != ref_end)
 	      *ref_iter++ = rtx_obj_reference (regno, flags,
 					       reg_raw_mode[regno], 0);
 	}
diff --git a/gcc/testsuite/gcc.target/arm/pr99596.c b/gcc/testsuite/gcc.target/arm/pr99596.c
new file mode 100644
index 00000000000..2b8b4c87e96
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr99596.c
@@ -0,0 +1,18 @@
+/* { dg-options "-Os -mtune=xscale" } */
+
+register int a asm("sp");
+extern int b;
+typedef struct {
+  long c[16 * 8 / 32];
+} d;
+int e;
+int f;
+int g;
+d h;
+int j(int, int, int, d);
+int i(void) {
+  for (;;) {
+    b &&j(e, f, g, h);
+    j(e, f, g, h);
+  }
+}