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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/176083">176083</a></dd>
<dt>Author</dt> <dd>benjamin@webkit.org</dd>
<dt>Date</dt> <dd>2014-11-13 12:43:32 -0800 (Thu, 13 Nov 2014)</dd>
</dl>
<h3>Log Message</h3>
<pre>ARMv7(s) Assembler: LDRH with immediate offset is loading from the wrong offset
https://bugs.webkit.org/show_bug.cgi?id=136914
Reviewed by Michael Saboff.
TLDR: the immediate offset of half-word load was divided by 2.
Story time: So I started getting those weird reports of :nth-child() behaving bizarrely
on ARMv7 and ARMv7s. To make things worse, the behavior changes depending on style updates.
I started looking the disassembly on the tests cases...
The first thing I noticed was that the computation of An+B looked wrong. For example,
in the case of n+6, the instruction should have been:
subs <a href="http://trac.webkit.org/projects/webkit/changeset/1">r1</a>, <a href="http://trac.webkit.org/projects/webkit/changeset/1">r1</a>, #6
but was
subs <a href="http://trac.webkit.org/projects/webkit/changeset/1">r1</a>, <a href="http://trac.webkit.org/projects/webkit/changeset/1">r1</a>, #2
After spending a lot of time trying to find the error in the assembler, I discovered
the problem was not real, but just a bug in the disassembler.
This is the first fix: ARMv7DOpcodeAddSubtractImmediate3's immediate3() was truncating
the value to 2 bits instead of 3 bits.
The disassembler being fixed, I still have no lead on the weird bug. Some disassembly later,
I realize the LDRH instruction is not decoded at all. The reason is that both LDRH and STRH
were under the umbrella ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord but the pattern
only matched SRTH.
I fix that next, ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord is split into
ARMv7DOpcodeStoreRegisterImmediateHalfWord and ARMv7DOpcodeLoadRegisterImmediateHalfWord,
each with their own pattern and their instruction group.
Now that I can see the LDRHs correctly, there is something fishy about them, their offset
is way too small for the data I load.
This time, looking at the binary, the generated code is indeed incorrect. It turns out that
the ARMv7 assembler shifted the offset of half-word load as if they were byte load: divided by 4.
As a result, all the load of half-words with more than zero offset were loading
values with a smaller offset than what they should have.
That being fixed, I dump the assembly: still wrong. I am ready to throw my keyboard through
my screen at that point.
Looking at the disassembler, there is yet again a bug. The computation of the scale() adjustment
of the offset was incorrect for anything but word loads.
I replaced it by a switch-case to make it explicit.
STRH is likely incorrect too. I'll fix that in a follow up, I want to survey all the 16 bits cases
that are not directly used by the CSS JIT.
* assembler/ARMv7Assembler.h:
(JSC::ARMv7Assembler::ldrh):
Fix the immediate scaling. Add an assertion to make sure the alignment of the input is correct.
* disassembler/ARMv7/ARMv7DOpcode.cpp:
(JSC::ARMv7Disassembler::ARMv7DOpcodeLoadStoreRegisterImmediate::scale):
Fix the scaling code. Just hardcode instruction-to-scale table.
* disassembler/ARMv7/ARMv7DOpcode.h:
(JSC::ARMv7Disassembler::ARMv7DOpcodeAddSubtractImmediate3::immediate3):
The mask for a 3 bits immediate is not 3 :)
(JSC::ARMv7Disassembler::ARMv7DOpcodeLoadStoreRegisterImmediate::scale): Deleted.</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoreassemblerARMv7Assemblerh">trunk/Source/JavaScriptCore/assembler/ARMv7Assembler.h</a></li>
<li><a href="#trunkSourceJavaScriptCoredisassemblerARMv7ARMv7DOpcodecpp">trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.cpp</a></li>
<li><a href="#trunkSourceJavaScriptCoredisassemblerARMv7ARMv7DOpcodeh">trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.h</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (176082 => 176083)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2014-11-13 20:15:33 UTC (rev 176082)
+++ trunk/Source/JavaScriptCore/ChangeLog 2014-11-13 20:43:32 UTC (rev 176083)
</span><span class="lines">@@ -1,3 +1,69 @@
</span><ins>+2014-11-13 Benjamin Poulain <benjamin@webkit.org>
+
+ ARMv7(s) Assembler: LDRH with immediate offset is loading from the wrong offset
+ https://bugs.webkit.org/show_bug.cgi?id=136914
+
+ Reviewed by Michael Saboff.
+
+ TLDR: the immediate offset of half-word load was divided by 2.
+
+ Story time: So I started getting those weird reports of :nth-child() behaving bizarrely
+ on ARMv7 and ARMv7s. To make things worse, the behavior changes depending on style updates.
+
+ I started looking the disassembly on the tests cases...
+
+ The first thing I noticed was that the computation of An+B looked wrong. For example,
+ in the case of n+6, the instruction should have been:
+ subs r1, r1, #6
+ but was
+ subs r1, r1, #2
+
+ After spending a lot of time trying to find the error in the assembler, I discovered
+ the problem was not real, but just a bug in the disassembler.
+ This is the first fix: ARMv7DOpcodeAddSubtractImmediate3's immediate3() was truncating
+ the value to 2 bits instead of 3 bits.
+
+ The disassembler being fixed, I still have no lead on the weird bug. Some disassembly later,
+ I realize the LDRH instruction is not decoded at all. The reason is that both LDRH and STRH
+ were under the umbrella ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord but the pattern
+ only matched SRTH.
+
+ I fix that next, ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord is split into
+ ARMv7DOpcodeStoreRegisterImmediateHalfWord and ARMv7DOpcodeLoadRegisterImmediateHalfWord,
+ each with their own pattern and their instruction group.
+
+ Now that I can see the LDRHs correctly, there is something fishy about them, their offset
+ is way too small for the data I load.
+
+ This time, looking at the binary, the generated code is indeed incorrect. It turns out that
+ the ARMv7 assembler shifted the offset of half-word load as if they were byte load: divided by 4.
+ As a result, all the load of half-words with more than zero offset were loading
+ values with a smaller offset than what they should have.
+
+ That being fixed, I dump the assembly: still wrong. I am ready to throw my keyboard through
+ my screen at that point.
+
+ Looking at the disassembler, there is yet again a bug. The computation of the scale() adjustment
+ of the offset was incorrect for anything but word loads.
+ I replaced it by a switch-case to make it explicit.
+
+ STRH is likely incorrect too. I'll fix that in a follow up, I want to survey all the 16 bits cases
+ that are not directly used by the CSS JIT.
+
+ * assembler/ARMv7Assembler.h:
+ (JSC::ARMv7Assembler::ldrh):
+ Fix the immediate scaling. Add an assertion to make sure the alignment of the input is correct.
+
+ * disassembler/ARMv7/ARMv7DOpcode.cpp:
+ (JSC::ARMv7Disassembler::ARMv7DOpcodeLoadStoreRegisterImmediate::scale):
+ Fix the scaling code. Just hardcode instruction-to-scale table.
+
+ * disassembler/ARMv7/ARMv7DOpcode.h:
+ (JSC::ARMv7Disassembler::ARMv7DOpcodeAddSubtractImmediate3::immediate3):
+ The mask for a 3 bits immediate is not 3 :)
+
+ (JSC::ARMv7Disassembler::ARMv7DOpcodeLoadStoreRegisterImmediate::scale): Deleted.
+
</ins><span class="cx"> 2014-11-13 Andreas Kling <akling@apple.com>
</span><span class="cx">
</span><span class="cx"> Generate put_by_id for bracket assignment with constant string subscript.
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoreassemblerARMv7Assemblerh"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/assembler/ARMv7Assembler.h (176082 => 176083)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/assembler/ARMv7Assembler.h 2014-11-13 20:15:33 UTC (rev 176082)
+++ trunk/Source/JavaScriptCore/assembler/ARMv7Assembler.h 2014-11-13 20:43:32 UTC (rev 176083)
</span><span class="lines">@@ -1155,9 +1155,10 @@
</span><span class="cx"> {
</span><span class="cx"> ASSERT(rn != ARMRegisters::pc); // LDR (literal)
</span><span class="cx"> ASSERT(imm.isUInt12());
</span><ins>+ ASSERT(!(imm.isUInt12() & 1));
</ins><span class="cx">
</span><span class="cx"> if (!((rt | rn) & 8) && imm.isUInt6())
</span><del>- m_formatter.oneWordOp5Imm5Reg3Reg3(OP_LDRH_imm_T1, imm.getUInt6() >> 2, rn, rt);
</del><ins>+ m_formatter.oneWordOp5Imm5Reg3Reg3(OP_LDRH_imm_T1, imm.getUInt6() >> 1, rn, rt);
</ins><span class="cx"> else
</span><span class="cx"> m_formatter.twoWordOp12Reg4Reg4Imm12(OP_LDRH_imm_T2, rn, rt, imm.getUInt12());
</span><span class="cx"> }
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoredisassemblerARMv7ARMv7DOpcodecpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.cpp (176082 => 176083)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.cpp 2014-11-13 20:15:33 UTC (rev 176082)
+++ trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.cpp 2014-11-13 20:43:32 UTC (rev 176083)
</span><span class="lines">@@ -91,8 +91,8 @@
</span><span class="cx"> OPCODE_GROUP_ENTRY(0xd, ARMv7DOpcodeLoadStoreRegisterImmediateWordAndByte),
</span><span class="cx"> OPCODE_GROUP_ENTRY(0xe, ARMv7DOpcodeLoadStoreRegisterImmediateWordAndByte),
</span><span class="cx"> OPCODE_GROUP_ENTRY(0xf, ARMv7DOpcodeLoadStoreRegisterImmediateWordAndByte),
</span><del>- OPCODE_GROUP_ENTRY(0x10, ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord),
- OPCODE_GROUP_ENTRY(0x11, ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord),
</del><ins>+ OPCODE_GROUP_ENTRY(0x10, ARMv7DOpcodeStoreRegisterImmediateHalfWord),
+ OPCODE_GROUP_ENTRY(0x11, ARMv7DOpcodeLoadRegisterImmediateHalfWord),
</ins><span class="cx"> OPCODE_GROUP_ENTRY(0x12, ARMv7DOpcodeLoadStoreRegisterSPRelative),
</span><span class="cx"> OPCODE_GROUP_ENTRY(0x13, ARMv7DOpcodeLoadStoreRegisterSPRelative),
</span><span class="cx"> OPCODE_GROUP_ENTRY(0x14, ARMv7DOpcodeGeneratePCRelativeAddress),
</span><span class="lines">@@ -524,6 +524,25 @@
</span><span class="cx"> return m_formatBuffer;
</span><span class="cx"> }
</span><span class="cx">
</span><ins>+unsigned ARMv7DOpcodeLoadStoreRegisterImmediate::scale()
+{
+ switch (op()) {
+ case 0:
+ case 1:
+ return 2;
+ case 2:
+ case 3:
+ return 0;
+ case 4:
+ case 5:
+ return 1;
+ default:
+ break;
+ }
+ ASSERT_NOT_REACHED();
+ return 0;
+}
+
</ins><span class="cx"> const char* const ARMv7DOpcodeLoadStoreRegisterOffsetT1::s_opNames[8] = {
</span><span class="cx"> "str", "strh", "strb", "ldrsb", "ldr", "ldrh", "ldrb", "ldrsh"
</span><span class="cx"> };
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoredisassemblerARMv7ARMv7DOpcodeh"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.h (176082 => 176083)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.h 2014-11-13 20:15:33 UTC (rev 176082)
+++ trunk/Source/JavaScriptCore/disassembler/ARMv7/ARMv7DOpcode.h 2014-11-13 20:43:32 UTC (rev 176083)
</span><span class="lines">@@ -275,7 +275,7 @@
</span><span class="cx"> const char* opName() { return s_opNames[op()]; }
</span><span class="cx">
</span><span class="cx"> unsigned op() { return (m_opcode >> 9) & 0x1; }
</span><del>- unsigned immediate3() { return (m_opcode >> 6) & 0x3; }
</del><ins>+ unsigned immediate3() { return (m_opcode >> 6) & 0x7; }
</ins><span class="cx"> unsigned rn() { return (m_opcode >> 3) & 0x7; }
</span><span class="cx"> };
</span><span class="cx">
</span><span class="lines">@@ -441,7 +441,7 @@
</span><span class="cx"> unsigned immediate5() { return (m_opcode >> 6) & 0x01f; }
</span><span class="cx"> unsigned rn() { return (m_opcode >> 3) & 0x7; }
</span><span class="cx"> unsigned rt() { return m_opcode & 0x7; }
</span><del>- unsigned scale() { return 2 - (op() >> 1); }
</del><ins>+ unsigned scale();
</ins><span class="cx"> };
</span><span class="cx">
</span><span class="cx"> class ARMv7DOpcodeLoadStoreRegisterImmediateWordAndByte : public ARMv7DOpcodeLoadStoreRegisterImmediate {
</span><span class="lines">@@ -452,7 +452,7 @@
</span><span class="cx"> DEFINE_STATIC_FORMAT16(ARMv7DOpcodeLoadStoreRegisterImmediate, thisObj);
</span><span class="cx"> };
</span><span class="cx">
</span><del>-class ARMv7DOpcodeLoadStoreRegisterImmediateHalfWord : public ARMv7DOpcodeLoadStoreRegisterImmediate {
</del><ins>+class ARMv7DOpcodeStoreRegisterImmediateHalfWord : public ARMv7DOpcodeLoadStoreRegisterImmediate {
</ins><span class="cx"> public:
</span><span class="cx"> static const uint16_t s_mask = 0xf800;
</span><span class="cx"> static const uint16_t s_pattern = 0x8000;
</span><span class="lines">@@ -460,6 +460,14 @@
</span><span class="cx"> DEFINE_STATIC_FORMAT16(ARMv7DOpcodeLoadStoreRegisterImmediate, thisObj);
</span><span class="cx"> };
</span><span class="cx">
</span><ins>+class ARMv7DOpcodeLoadRegisterImmediateHalfWord : public ARMv7DOpcodeLoadStoreRegisterImmediate {
+public:
+ static const uint16_t s_mask = 0xf800;
+ static const uint16_t s_pattern = 0x8800;
+
+ DEFINE_STATIC_FORMAT16(ARMv7DOpcodeLoadStoreRegisterImmediate, thisObj);
+};
+
</ins><span class="cx"> class ARMv7DOpcodeLoadStoreRegisterOffsetT1 : public ARMv7D16BitOpcode {
</span><span class="cx"> private:
</span><span class="cx"> static const char* const s_opNames[8];
</span></span></pre>
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