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[ARM] mno-pic-data-is-text-relative & msingle-pic-base


This patch comes from an off-list conversation between Joey & me. The context is with RTOSs not all singing & dancing dynamic objects and OSes.

currently, the documentation for -mno-pic-data-is-text-relative (-mno-PDITR) says
'Assume that each data segments are relative to text segment at load time.
 Therefore, it permits addressing data using PC-relative operations.
 This option is on by default for targets other than VxWorks RTP.'

However, if you use just this option, you still end up with a pic-register init sequence that presumes a fixed mapping. That's a surprise. Joey tells me its expected use is with -msingle-pic-base (-mSPB), which reserves a global register to point at the (single) GOT. That's what I had expected the -mno-PDITR option to have implied.

Apparently there are legitimate reasons one might want the -mno-PDITR behaviour without -mSPB. I don't know what those are, perhaps Joey could clarify?

Anyway, IMHO that is the rare case and the more common case is that one would want to have -mnoPDITR imply -mSPB. (The reverse probably doesn't apply.)

This patch does 3 things.
1) have -mno-PDITR imply -mSPB, unless one has explictly provided -m[no-]SPB.
2) clarified  the -m[no-]PDITR documentation.
3) Added some testcases -- there didn't appear to be any.

ok?

nathan
2016-05-09  Nathan Sidwell  <nathan@acm.org>

	gcc/
	* config/arm/arm.c (arm_option_override): Set MASK_SINGLE_PIC_BASE
	when -mno-pic-data-is-text-relative is in effect, by default.
	* doc/invoke.texi (mpic-data-is-text-relative): Document new
	behavior and clarify.

	gcc/testsuite/
	* gcc.target/arm/data-rel-1.c: New.
	* gcc.target/arm/data-rel-2.c: New.
	* gcc.target/arm/data-rel-3.c: New.

Index: config/arm/arm.c
===================================================================
--- config/arm/arm.c	(revision 235980)
+++ config/arm/arm.c	(working copy)
@@ -3298,6 +3298,20 @@ arm_option_override (void)
 	}
     }
 
+  if (TARGET_VXWORKS_RTP)
+    {
+      if (!global_options_set.x_arm_pic_data_is_text_relative)
+	arm_pic_data_is_text_relative = 0;
+    }
+  else if (flag_pic
+	   && !arm_pic_data_is_text_relative
+	   && !(global_options_set.x_target_flags & MASK_SINGLE_PIC_BASE))
+    /* When text & data segments don't have a fixed displacement, the
+       intended use is with a single, read only, pic base register.
+       Unless the user explicitly requested not to do that, set
+       it.  */
+    target_flags |= MASK_SINGLE_PIC_BASE;
+
   /* If stack checking is disabled, we can use r10 as the PIC register,
      which keeps r9 available.  The EABI specifies r9 as the PIC register.  */
   if (flag_pic && TARGET_SINGLE_PIC_BASE)
@@ -3329,10 +3343,6 @@ arm_option_override (void)
 	arm_pic_register = pic_register;
     }
 
-  if (TARGET_VXWORKS_RTP
-      && !global_options_set.x_arm_pic_data_is_text_relative)
-    arm_pic_data_is_text_relative = 0;
-
   /* Enable -mfix-cortex-m3-ldrd by default for Cortex-M3 cores.  */
   if (fix_cm3_ldrd == 2)
     {
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 235980)
+++ doc/invoke.texi	(working copy)
@@ -14197,9 +14197,12 @@ otherwise the default is @samp{R10}.
 
 @item -mpic-data-is-text-relative
 @opindex mpic-data-is-text-relative
-Assume that each data segments are relative to text segment at load time.
-Therefore, it permits addressing data using PC-relative operations.
-This option is on by default for targets other than VxWorks RTP.
+Assume that the displacement between the text and data segments is fixed
+at static link time.  This permits using PC-relative addressing
+operations to access data known to be in the data segment.  For
+non-VxWorks RTP targets, this option is enabled by default.  When
+disabled on such targets, it will enable @option{-msingle-pic-base} by
+default.
 
 @item -mpoke-function-name
 @opindex mpoke-function-name
Index: testsuite/gcc.target/arm/data-rel-1.c
===================================================================
--- testsuite/gcc.target/arm/data-rel-1.c	(nonexistent)
+++ testsuite/gcc.target/arm/data-rel-1.c	(working copy)
@@ -0,0 +1,12 @@
+/* { dg-options "-fPIC -mno-pic-data-is-text-relative" } */
+/* { dg-final { scan-assembler-not "j-\\(.LPIC"  } } */
+/* { dg-final { scan-assembler-not "_GLOBAL_OFFSET_TABLE_-\\(.LPIC" } } */
+/* { dg-final { scan-assembler "j\\(GOT\\)" } } */
+/* { dg-final { scan-assembler "(ldr|mov)\tr\[0-9\]+, \\\[?r9" } } */
+
+static int j;
+
+int *Foo ()
+{
+  return &j;
+}
Index: testsuite/gcc.target/arm/data-rel-2.c
===================================================================
--- testsuite/gcc.target/arm/data-rel-2.c	(nonexistent)
+++ testsuite/gcc.target/arm/data-rel-2.c	(working copy)
@@ -0,0 +1,11 @@
+/* { dg-options "-fPIC -mno-pic-data-is-text-relative -mno-single-pic-base" } */
+/* { dg-final { scan-assembler-not "j-\\(.LPIC"  } } */
+/* { dg-final { scan-assembler "_GLOBAL_OFFSET_TABLE_-\\(.LPIC" } } */
+/* { dg-final { scan-assembler "j\\(GOT\\)" } } */
+
+static int j;
+
+int *Foo ()
+{
+  return &j;
+}
Index: testsuite/gcc.target/arm/data-rel-3.c
===================================================================
--- testsuite/gcc.target/arm/data-rel-3.c	(nonexistent)
+++ testsuite/gcc.target/arm/data-rel-3.c	(working copy)
@@ -0,0 +1,11 @@
+/* { dg-options "-fPIC -mpic-data-is-text-relative" } */
+/* { dg-final { scan-assembler "j-\\(.LPIC"  } } */
+/* { dg-final { scan-assembler-not "_GLOBAL_OFFSET_TABLE_-\\(.LPIC" } } */
+/* { dg-final { scan-assembler-not "j\\(GOT\\)" } } */
+
+static int j;
+
+int *Foo ()
+{
+  return &j;
+}

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