[PATCH, GCC/ARM] Remove ARMv8-M code for D17-D31

[Thomas Preudhomme <thomas.preudhomme@foss.arm.com>]

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Hi,

Function cmse_nonsecure_entry_clear_before_return has code to deal with
high VFP register (D16-D31) while ARMv8-M Baseline and Mainline both do
not support more than 16 double VFP registers (D0-D15). This makes this
security-sensitive code harder to read for not much benefit since
libcall for cmse_nonsecure_call functions do not deal with those high
VFP registers anyway.

This commit gets rid of this code for simplicity and fixes 2 issues in
the same function:

- stop the first loop when reaching maxregno to avoid dealing with VFP
   registers if targetting Thumb-1 or using -mfloat-abi=soft
- include maxregno in that loop

ChangeLog entry is as follows:

*** gcc/ChangeLog ***

2017-06-13  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* config/arm/arm.c (arm_option_override): Forbid ARMv8-M Security
	Extensions with more than 16 double VFP registers.
	(cmse_nonsecure_entry_clear_before_return): Remove second entry of
	to_clear_mask and all code related to it and make the remaining
	entry a 64-bit scalar integer variable and adapt code accordingly.

Testing: Testsuite shows no regression when run for ARMv8-M Baseline and
ARMv8-M Mainline.

Is this ok for trunk?

Best regards,

Thomas

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diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 259597d8890ee84c5bd92b12b6f9f6521c8dcd2e..60a4d1f46765d285de469f51fbb5a0ad76d56d9b 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -3620,6 +3620,11 @@ arm_option_override (void)
   if (use_cmse && !arm_arch_cmse)
     error ("target CPU does not support ARMv8-M Security Extensions");
 
+  /* We don't clear D16-D31 VFP registers for cmse_nonsecure_call functions
+     and ARMv8-M Baseline and Mainline do not allow such configuration.  */
+  if (use_cmse && LAST_VFP_REGNUM > LAST_LO_VFP_REGNUM)
+    error ("ARMv8-M Security Extensions incompatible with selected FPU");
+
   /* Disable scheduling fusion by default if it's not armv7 processor
      or doesn't prefer ldrd/strd.  */
   if (flag_schedule_fusion == 2
@@ -24996,15 +25001,15 @@ thumb1_expand_prologue (void)
 void
 cmse_nonsecure_entry_clear_before_return (void)
 {
-  uint64_t to_clear_mask[2];
+  uint64_t to_clear_mask;
   uint32_t padding_bits_to_clear = 0;
   uint32_t * padding_bits_to_clear_ptr = &padding_bits_to_clear;
   int regno, maxregno = IP_REGNUM;
   tree result_type;
   rtx result_rtl;
 
-  to_clear_mask[0] = (1ULL << (NUM_ARG_REGS)) - 1;
-  to_clear_mask[0] |= (1ULL << IP_REGNUM);
+  to_clear_mask = (1ULL << (NUM_ARG_REGS)) - 1;
+  to_clear_mask |= (1ULL << IP_REGNUM);
 
   /* If we are not dealing with -mfloat-abi=soft we will need to clear VFP
      registers.  We also check that TARGET_HARD_FLOAT and !TARGET_THUMB1 hold
@@ -25015,23 +25020,22 @@ cmse_nonsecure_entry_clear_before_return (void)
       maxregno = LAST_VFP_REGNUM;
 
       float_mask &= ~((1ULL << FIRST_VFP_REGNUM) - 1);
-      to_clear_mask[0] |= float_mask;
-
-      float_mask = (1ULL << (maxregno - 63)) - 1;
-      to_clear_mask[1] = float_mask;
+      to_clear_mask |= float_mask;
 
       /* Make sure we don't clear the two scratch registers used to clear the
 	 relevant FPSCR bits in output_return_instruction.  */
       emit_use (gen_rtx_REG (SImode, IP_REGNUM));
-      to_clear_mask[0] &= ~(1ULL << IP_REGNUM);
+      to_clear_mask &= ~(1ULL << IP_REGNUM);
       emit_use (gen_rtx_REG (SImode, 4));
-      to_clear_mask[0] &= ~(1ULL << 4);
+      to_clear_mask &= ~(1ULL << 4);
     }
 
+  gcc_assert ((unsigned) maxregno <= sizeof (to_clear_mask) * __CHAR_BIT__);
+
   /* If the user has defined registers to be caller saved, these are no longer
      restored by the function before returning and must thus be cleared for
      security purposes.  */
-  for (regno = NUM_ARG_REGS; regno < LAST_VFP_REGNUM; regno++)
+  for (regno = NUM_ARG_REGS; regno <= maxregno; regno++)
     {
       /* We do not touch registers that can be used to pass arguments as per
 	 the AAPCS, since these should never be made callee-saved by user
@@ -25041,7 +25045,7 @@ cmse_nonsecure_entry_clear_before_return (void)
       if (IN_RANGE (regno, IP_REGNUM, PC_REGNUM))
 	continue;
       if (call_used_regs[regno])
-	to_clear_mask[regno / 64] |= (1ULL << (regno % 64));
+	to_clear_mask |= (1ULL << regno);
     }
 
   /* Make sure we do not clear the registers used to return the result in.  */
@@ -25052,7 +25056,7 @@ cmse_nonsecure_entry_clear_before_return (void)
 
       /* No need to check that we return in registers, because we don't
 	 support returning on stack yet.  */
-      to_clear_mask[0]
+      to_clear_mask
 	&= ~compute_not_to_clear_mask (result_type, result_rtl, 0,
 				       padding_bits_to_clear_ptr);
     }
@@ -25063,7 +25067,7 @@ cmse_nonsecure_entry_clear_before_return (void)
       /* Padding bits to clear is not 0 so we know we are dealing with
 	 returning a composite type, which only uses r0.  Let's make sure that
 	 r1-r3 is cleared too, we will use r1 as a scratch register.  */
-      gcc_assert ((to_clear_mask[0] & 0xe) == 0xe);
+      gcc_assert ((to_clear_mask & 0xe) == 0xe);
 
       reg_rtx = gen_rtx_REG (SImode, R1_REGNUM);
 
@@ -25085,7 +25089,7 @@ cmse_nonsecure_entry_clear_before_return (void)
 
   for (regno = R0_REGNUM; regno <= maxregno; regno++)
     {
-      if (!(to_clear_mask[regno / 64] & (1ULL << (regno % 64))))
+      if (!(to_clear_mask & (1ULL << regno)))
 	continue;
 
       if (IS_VFP_REGNUM (regno))
@@ -25094,7 +25098,7 @@ cmse_nonsecure_entry_clear_before_return (void)
 	     be cleared, use vmov.  */
 	  if (TARGET_VFP_DOUBLE
 	      && VFP_REGNO_OK_FOR_DOUBLE (regno)
-	      && to_clear_mask[regno / 64] & (1ULL << ((regno % 64) + 1)))
+	      && to_clear_mask & (1ULL << (regno + 1)))
 	    {
 	      emit_move_insn (gen_rtx_REG (DFmode, regno),
 			      CONST1_RTX (DFmode));