[PATCH] arm: Warn if IRQ handler is not compiled with -mgeneral-regs-only [PR target/94743]

[Christophe Lyon]

The interrupt attribute does not guarantee that the FP registers are
saved, which can result in problems difficult to debug.

Saving the FP registers and status registers can be a large penalty,
so it's probably not desirable to do that all the time.

If the handler calls other functions, we'd likely need to save all of
them, for lack of knowledge of which registers they actually use.

This is even more obscure for the end-user when the compiler inserts
calls to helper functions such as memcpy (some multilibs do use FP
registers to speed it up).

In the PR, we discussed adding routines in libgcc to save the FP
context and saving only locally-clobbered FP registers, but I think
this is too intrusive for stage 4.

In the mean time, emit a warning to suggest re-compiling with
-mgeneral-regs-only. Note that this can lead to errors if the code
uses floating-point and -mfloat-abi=hard, eg:
argument of type 'double' not permitted with -mgeneral-regs-only

This can be troublesome for the user, but at least this would make
them aware of the latent issue.

The patch adds two testcases:
- pr94734-1.c checks that a warning is emitted. One function can make
  implicit calls to runtime floating-point routines, the other one
  doesn't. We can improve the diagnostic later not to warn in the
  second case.

- pr94734-2.c checks that no warning is emitted when using
  -mgeneral-regs-only.

2020-04-29  Christophe Lyon  <christophe.lyon@linaro.org>

	PR target/94743
	gcc/
	* config/arm/arm.c (arm_handle_isr_attribute): Warn if
	-mgeneral-regs-only is not used.

	gcc/testsuite/
	* gcc.target/arm/pr94743-1.c: New test.
	* gcc.target/arm/pr94743-2.c: New test.
---
 gcc/config/arm/arm.c                     |  5 +++++
 gcc/testsuite/gcc.target/arm/pr94743-1.c | 20 ++++++++++++++++++++
 gcc/testsuite/gcc.target/arm/pr94743-2.c | 17 +++++++++++++++++
 3 files changed, 42 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/arm/pr94743-1.c
 create mode 100644 gcc/testsuite/gcc.target/arm/pr94743-2.c

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 6a6e804..34aad1d 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -7176,6 +7176,11 @@ arm_handle_isr_attribute (tree *node, tree name, tree args, int flags,
 		   name);
 	  *no_add_attrs = true;
 	}
+      else if (TARGET_VFP_BASE)
+	{
+	  warning (OPT_Wattributes, "FP registers might be clobbered despite %qE attribute: compile with -mgeneral-regs-only",
+		   name);
+	}
       /* FIXME: the argument if any is checked for type attributes;
 	 should it be checked for decl ones?  */
     }
diff --git a/gcc/testsuite/gcc.target/arm/pr94743-1.c b/gcc/testsuite/gcc.target/arm/pr94743-1.c
new file mode 100644
index 0000000..67700c6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr94743-1.c
@@ -0,0 +1,20 @@
+/* PR target/94743 */
+/* { dg-do compile } */
+
+typedef struct {
+  double fpdata[32];
+} dummy_t;
+
+dummy_t global_d;
+dummy_t global_d1;
+
+__attribute__ ((interrupt("IRQ"))) void IRQ_HDLR_Test(void)
+{ /* { dg-warning { FP registers might be clobbered despite 'interrupt' attribute: compile with -mgeneral-regs-only} "" { target *-*-* } . } */
+  global_d.fpdata[3] += global_d.fpdata[3] * global_d1.fpdata[3];
+}
+
+
+__attribute__ ((interrupt("IRQ"))) void IRQ_HDLR_Test2(void)
+{ /* { dg-warning { FP registers might be clobbered despite 'interrupt' attribute: compile with -mgeneral-regs-only} "" { target *-*-* } . } */
+  global_d.fpdata[3] = 1.0;
+}
diff --git a/gcc/testsuite/gcc.target/arm/pr94743-2.c b/gcc/testsuite/gcc.target/arm/pr94743-2.c
new file mode 100644
index 0000000..745fd36
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr94743-2.c
@@ -0,0 +1,17 @@
+/* PR target/94743 */
+/* { dg-do compile } */
+/* { dg-options "-mgeneral-regs-only" } */
+
+typedef struct {
+  /* Do not use floating-point types, which are not be compatible with
+     -mgeneral-regs-only under -mfloat-abi=hard */
+  int fpdata[32];
+} dummy_t;
+
+dummy_t global_d;
+dummy_t global_d1;
+
+__attribute__ ((interrupt("IRQ"))) void IRQ_HDLR_Test(void)
+{
+  global_d.fpdata[3] += global_d.fpdata[3] * global_d1.fpdata[3];
+}
-- 
2.7.4