Changeset 214969 in webkit

Timestamp:

Apr 5, 2017 2:00:17 PM (7 years ago)

Author:

commit-queue@webkit.org

Message:

Do not use BLX for immediates (ARM-32)

​https://bugs.webkit.org/show_bug.cgi?id=170351

Patch by Guilherme Iscaro <​iscaro@profusion.mobi> on 2017-04-05
Reviewed by Mark Lam.

Currently the offline asm generator for 32-bit ARM code translates the
'call' meta-instruction (which may be found in LowLevelInterpreter.asm
and friends) to the ARM's BLX instrunction. The BLX instruction may be
used for labels (immediates) and registers and one side effect of BLX
is that it may switch the processor's instruction set.
A 'BLX register' instruction will change/remain the processor state to
ARM if the register_bit[0] is set to 0 or change/remain to Thumb if
register_bit[0] is set to 1. However, a 'BLX label' instruction will
always switch the processor state. It switches ARM to thumb and vice-versa.
This behaviour is unwanted, since the C++ code and the offlineasm generated code
are both compiled using the same instruction set, thus a instruction
set change will likely produce a crash. In order to fix the problem the
BL instruction can be used for labels. It will branch just like BLX,
but it won't change the instruction set. It's important to note that
Darwin is not affected by this problem, thus to minimize the impact of
this change the BL instruction will only be used on non-darwin targets.

BLX reference: ​http://infocenter.arm.com/help/topic/com.arm.doc.dui0489i/CIHBJCDC.html?resultof=%22%62%6c%78%22%20

• offlineasm/arm.rb:

Location:

trunk/Source/JavaScriptCore

Files:

• ChangeLog (modified) (1 diff)
• offlineasm/arm.rb (modified) (2 diffs)

trunk/Source/JavaScriptCore/ChangeLog

@@ +1 @@
+2017-04-05  Guilherme Iscaro  <iscaro@profusion.mobi>
+
+        Do not use BLX for immediates (ARM-32)
+
+        https://bugs.webkit.org/show_bug.cgi?id=170351
+
+        Reviewed by Mark Lam.
+
+        Currently the offline asm generator for 32-bit ARM code translates the
+        'call' meta-instruction (which may be found in LowLevelInterpreter.asm
+        and friends) to the ARM's BLX instrunction. The BLX instruction may be
+        used for labels (immediates) and registers and one side effect of BLX
+        is that it may switch the processor's instruction set.
+        A 'BLX register' instruction will change/remain the processor state to
+        ARM if the  register_bit[0] is set to 0 or change/remain to Thumb if
+        register_bit[0] is set to 1. However, a 'BLX label' instruction will
+        always switch the processor state. It switches ARM to thumb and vice-versa.
+        This behaviour is unwanted, since the C++ code and the offlineasm generated code
+        are both compiled using the same instruction set, thus a instruction
+        set change will likely produce a crash. In order to fix the problem the
+        BL instruction can be used for labels. It will branch just like BLX,
+        but it won't change the instruction set. It's important to note that
+        Darwin is not affected by this problem, thus to minimize the impact of
+        this change the BL instruction will only be used on non-darwin targets.
+
+        BLX reference: http://infocenter.arm.com/help/topic/com.arm.doc.dui0489i/CIHBJCDC.html?resultof=%22%62%6c%78%22%20
+
+        * offlineasm/arm.rb:
+
 2017-04-05  Keith Miller  <keith_miller@apple.com>
 

trunk/Source/JavaScriptCore/offlineasm/arm.rb

@@ -95 +95 @@
 ARM_EXTRA_FPRS = [SpecialRegister.new("d7")]
 ARM_SCRATCH_FPR = SpecialRegister.new("d6")
+OS_DARWIN = ((RUBY_PLATFORM =~ /darwin/i) != nil)
 
 def armMoveImmediate(value, register)
@@ -569 +570 @@
         when "call"
             if operands[0].label?
-                $asm.puts "blx #{operands[0].asmLabel}"
+                if OS_DARWIN
+                    $asm.puts "blx #{operands[0].asmLabel}"
+                else
+                    $asm.puts "bl #{operands[0].asmLabel}"
+                end
             else
                 $asm.puts "blx #{operands[0].armOperand}"