https://sourceware.org/bugzilla/show_bug.cgi?id=24642
Bug ID: 24642
Summary: Gold missing armv8 from may_use_v5t_interworking()
results in less efficient v4t stubs
Product: binutils
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gold
Assignee: ccoutant at gmail dot com
Reporter: peter.smith at linaro dot org
CC: ian at airs dot com
Target Milestone: ---
In arm.cc the code that permits v5t interworking instructions is missing a case
for TAG_CPU_ARCH_V8 and the various V8_M mainline and baseline. As
--fix-arm1176 is the default this means that gold is using v4t stubs for arm v8
cpus. This is not a problem for correctness but v4t stubs and the non-use of
BLX is likely to have a measurable code-size and performance impact.
This can be worked around with -fno-fix-arm1176 but it should be simple to fix.
// Whether we have v5T interworking instructions available.
bool
may_use_v5t_interworking() const
{
Object_attribute* attr =
this->get_aeabi_object_attribute(elfcpp::Tag_CPU_arch);
int arch = attr->int_value();
if (parameters->options().fix_arm1176())
return (arch == elfcpp::TAG_CPU_ARCH_V6T2
|| arch == elfcpp::TAG_CPU_ARCH_V7
|| arch == elfcpp::TAG_CPU_ARCH_V6_M
|| arch == elfcpp::TAG_CPU_ARCH_V6S_M
|| arch == elfcpp::TAG_CPU_ARCH_V7E_M);
else
return (arch != elfcpp::TAG_CPU_ARCH_PRE_V4
&& arch != elfcpp::TAG_CPU_ARCH_V4
&& arch != elfcpp::TAG_CPU_ARCH_V4T);
}
A reproducer:
// call.c
extern void func1(void);
void func2(void) {
func1();
}
arm-linux-gnueabihf-gcc call.c -mthumb -o call.so --shared -fuse-ld=gold
-march=armv8-a -fpic
arm-linux-gnueabihf-objdump -d --no-show-raw-insn call.so
000004c0 <func2>:
4c0: push {r7, lr}
4c2: add r7, sp, #0
4c4: bl 4cc <func2+0xc>
4c8: nop
4ca: pop {r7, pc}
4cc: bx pc
4ce: nop ; (mov r8, r8)
4d0: stmia r0!, {}
4d2: b.n 14 <_init-0x344>
4d4: blx 40f5f0 <_end+0x40d5c3>
4d8: mrc2 15, 5, pc, cr4, cr15, {7}
You can see the "bx pc" followed by "nop" of the v4t stub.
The stmia is Arm instructions disassembled as Thumb because there is no mapping
symbol to inform the disassembler that the state has changed.
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