Hello!
Currently, we use %rbx as REAL_PIC_OFFSET_TABLE_REGNUM on x86_64.
Since this register gets marked as fixed reg in
ix86_conditional_register_usage, we get into troubles with insns that
use %rbx (cmpxchg, cpuid). According to x86_64 psABI, we are free to
use any register, so attached patch changes %rbx with %r15 (also
following the example in the psABI). This patch has no implications on
small code model (that doesn't use REAL_PIC_OFFSET_TABLE_REGNUM
anyway), but on medium and large code model fixes usage of cpuid.h
(please see PR 55712 [1]) and avoids a pair of xchgs around cmpxchg or
cpuid instructions.
Probably, we can also enhance ix86_select_alt_pic_regnum for x86_64,
but this is 4.9 material.
2012-12-24 Uros Bizjak <ubiz...@gmail.com>
* config/i386/i386.md (R14_REG, R15_REG): New constants.
* config/i386/i386.h (REAL_PIC_OFFSET_TABLE_REGNUM): Use R15_REG
for 64bit targets.
[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55712
Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
Uros.
Index: i386.h
===================================================================
--- i386.h (revision 194703)
+++ i386.h (working copy)
@@ -1173,7 +1173,7 @@
the pic register when possible. The change is visible after the
prologue has been emitted. */
-#define REAL_PIC_OFFSET_TABLE_REGNUM BX_REG
+#define REAL_PIC_OFFSET_TABLE_REGNUM (TARGET_64BIT ? R15_REG : BX_REG)
#define PIC_OFFSET_TABLE_REGNUM \
((TARGET_64BIT && ix86_cmodel == CM_SMALL_PIC) \
Index: i386.md
===================================================================
--- i386.md (revision 194703)
+++ i386.md (working copy)
@@ -301,6 +301,8 @@
(R11_REG 40)
(R12_REG 41)
(R13_REG 42)
+ (R14_REG 43)
+ (R15_REG 44)
(XMM8_REG 45)
(XMM9_REG 46)
(XMM10_REG 47)
