[PATCH v2] libc: arm: Implement setjmp GCC backwards compatibility.

Victor L. Do Nascimento victor.donascimento@arm.com
Fri Feb 3 11:15:26 GMT 2023

When compiling Newlib for arm targets with GCC 12.1 onward, the
passing of architecture extension information to the assembler is
automatic, making the use of .fpu and .arch_extension directives
in assembly files redundant.

With older versions of GCC, however, these directives must be
hard-coded into the `arm/setjmp.S' file to allow the assembly of
instructions concerning the storage and subsequent reloading of the
floating point registers to/from the jump buffer, respectively.

This patch conditionally adds the `.fpu vfpxd' and `.arch_extension
mve' directives based on compile-time preprocessor macros concerning
GCC version and target architectural features, such that both the
assembly and linking of setjmp.S succeeds for older versions of
 newlib/libc/machine/arm/setjmp.S | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/newlib/libc/machine/arm/setjmp.S b/newlib/libc/machine/arm/setjmp.S
index c615f2428..5401534cd 100644
--- a/newlib/libc/machine/arm/setjmp.S
+++ b/newlib/libc/machine/arm/setjmp.S
@@ -64,6 +64,28 @@
 	.syntax unified
+/*  GCC 12.1 and later will tell the assembler exactly which floating
+    point (or MVE) unit is required and we don't want to override
+    that.  Conversely, older versions of the compiler don't pass this
+    information so we need to enable the VFP version that is most
+    appropriate.  The choice here should support all suitable VFP
+    versions that the older toolchains can handle.  */
+#if __GNUC__ && __GNUC__ < 12
+/*  Ensure that FPU instructions are correctly compiled and, likewise,
+    the appropriate build attributes are added to the resulting object
+    file.  Check whether the MVE extension is present and whether
+    we have support for hardware floating point-operations.  VFPxd
+    covers all the cases we need in this file for hardware
+    floating-point and should be compatible with all required FPUs
+    that we need to support.  */
+# if __ARM_FP
+	.fpu vfpxd
+# endif
+	.arch_extension mve
+# endif
 /* ARMv6-M-like has to be implemented in Thumb mode.  */

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