gdb/arm: Handle lazy FPU state preservation

Read LSPEN, ASPEN and LSPACT bits from FPCCR and use them together
with FPCAR to identify if lazy FPU state preservation is active for
the current frame.  See "Lazy context save of FP state", in B1.5.7,
also ARM AN298, supported by Cortex-M4F architecture for details on
lazy FPU register stacking.  The same conditions are valid for other
Cortex-M cores with FPU.

This patch has been verified on a STM32F4-Discovery board by:
a) writing a non-zero value (lets use 0x1122334455667788 as an
   example) to all the D-registers in the main function
b) configured the SysTick to fire
c) in the SysTick_Handler, write some other value (lets use
   0x0022446688aaccee as an example) to one of the D-registers (D0 as
   an example) and then do "SVC #0"
d) in the SVC_Handler, write some other value (lets use
   0x0099aabbccddeeff) to one of the D-registers (D0 as an example)

In GDB, suspend the execution in the SVC_Handler function and compare
the value of the D-registers for the SVC_handler frame and the
SysTick_Handler frame.  With the patch, the value of the modified
D-register (D0) should be the new value (0x009..eff) on the
SVC_Handler frame, and the intermediate value (0x002..cee) for the
SysTick_Handler frame.  Now compare the D-register value for the
SysTick_Handler frame and the main frame.  The main frame should
have the initial value (0x112..788).

Signed-off-by: Torbjörn SVENSSON  <torbjorn.svensson@foss.st.com>
Signed-off-by: Yvan ROUX  <yvan.roux@foss.st.com>

2 files changed, 46 insertions, 17 deletions

diff --git a/gdb/arch/arm.h b/gdb/arch/arm.h
index 3675749..d384b95 100644
--- a/gdb/arch/arm.h
+++ b/gdb/arch/arm.h
@@ -115,7 +115,12 @@ enum system_register_address : CORE_ADDR
   /* M-profile Floating-Point Context Control Register address, defined in
      ARMv7-M (Section B3.2.2) and ARMv8-M (Section D1.2.99) reference
      manuals.  */
-  FPCCR = 0xe000ef34
+  FPCCR = 0xe000ef34,
+
+  /* M-profile Floating-Point Context Address Register address, defined in
+     ARMv7-M (Section B3.2.2) and ARMv8-M (Section D1.2.98) reference
+     manuals.  */
+  FPCAR = 0xe000ef38
 };
 
 /* Instruction condition field values.  */
diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 2810232..d357066 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -3588,27 +3588,48 @@ arm_m_exception_cache (struct frame_info *this_frame)
       if (extended_frame_used)
 	{
 	  ULONGEST fpccr;
+	  ULONGEST fpcar;
 
 	  /* Read FPCCR register.  */
 	  gdb_assert (safe_read_memory_unsigned_integer (FPCCR,
 							 ARM_INT_REGISTER_SIZE,
 							 byte_order, &fpccr));
-	  bool fpccr_ts = bit (fpccr, 26);
 
-	  /* This code does not take into account the lazy stacking, see "Lazy
-	     context save of FP state", in B1.5.7, also ARM AN298, supported
-	     by Cortex-M4F architecture.
-	     To fully handle this the FPCCR register (Floating-point Context
-	     Control Register) needs to be read out and the bits ASPEN and
-	     LSPEN could be checked to setup correct lazy stacked FP registers.
-	     This register is located at address 0xE000EF34.  */
+	  /* Read FPCAR register.  */
+	  if (!safe_read_memory_unsigned_integer (FPCAR, ARM_INT_REGISTER_SIZE,
+						  byte_order, &fpcar))
+	    {
+	      warning (_("Could not fetch FPCAR content. Further unwinding of "
+			 "FP register values will be unreliable."));
+	      fpcar = 0;
+	    }
+
+	  bool fpccr_aspen = bit (fpccr, 31);
+	  bool fpccr_lspen = bit (fpccr, 30);
+	  bool fpccr_ts = bit (fpccr, 26);
+	  bool fpccr_lspact = bit (fpccr, 0);
+
+	  /* The LSPEN and ASPEN bits indicate if the lazy state preservation
+	     for FP registers is enabled or disabled.  The LSPACT bit indicate,
+	     together with FPCAR, if the lazy state preservation feature is
+	     active for the current frame or for another frame.
+	     See "Lazy context save of FP state", in B1.5.7, also ARM AN298,
+	     supported by Cortex-M4F architecture for details.  */
+	  bool fpcar_points_to_this_frame = ((unwound_sp + sp_r0_offset + 0x20)
+					     == (fpcar & ~0x7));
+	  bool read_fp_regs_from_stack = (!(fpccr_aspen && fpccr_lspen
+					    && fpccr_lspact
+					    && fpcar_points_to_this_frame));
 
 	  /* Extended stack frame type used.  */
-	  CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x20;
-	  for (int i = 0; i < 8; i++)
+	  if (read_fp_regs_from_stack)
 	    {
-	      cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
-	      addr += 8;
+	      CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x20;
+	      for (int i = 0; i < 8; i++)
+		{
+		  cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
+		  addr += 8;
+		}
 	    }
 	  cache->saved_regs[ARM_FPSCR_REGNUM].set_addr (unwound_sp
 							+ sp_r0_offset + 0x60);
@@ -3617,11 +3638,14 @@ arm_m_exception_cache (struct frame_info *this_frame)
 	      && fpccr_ts)
 	    {
 	      /* Handle floating-point callee saved registers.  */
-	      addr = unwound_sp + sp_r0_offset + 0x68;
-	      for (int i = 8; i < 16; i++)
+	      if (read_fp_regs_from_stack)
 		{
-		  cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
-		  addr += 8;
+		  CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x68;
+		  for (int i = 8; i < 16; i++)
+		    {
+		      cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
+		      addr += 8;
+		    }
 		}
 
 	      arm_cache_set_active_sp_value (cache, tdep,