V4 JIT: teach regalloc about callee-saved registers.

Callee-saved registers won't get clobbered by a call, so any value
stored in it will survive the call.

Change-Id: I2b61b84617b01bf7ce7e81cd0119a6da689ee9d5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

2 files changed, 26 insertions, 9 deletions

diff --git a/src/qml/jit/qv4regalloc.cpp b/src/qml/jit/qv4regalloc.cpp
index e21676e520..6a2f5ca640 100644
--- a/src/qml/jit/qv4regalloc.cpp
+++ b/src/qml/jit/qv4regalloc.cpp
@@ -1280,19 +1280,23 @@ void RegisterAllocator::prepareRanges()
     const int regCount = _normalRegisters.size();
     _fixedRegisterRanges.reserve(regCount);
     for (int reg = 0; reg < regCount; ++reg) {
-        LifeTimeInterval *lti = cloneFixedInterval(reg, false, ltiWithCalls);
-        _fixedRegisterRanges.append(lti);
-        if (lti->isValid())
-            _active.append(lti);
+        if (_normalRegisters.at(reg)->isCallerSaved()) {
+            LifeTimeInterval *lti = cloneFixedInterval(reg, false, ltiWithCalls);
+            _fixedRegisterRanges.append(lti);
+            if (lti->isValid())
+                _active.append(lti);
+        }
     }
 
     const int fpRegCount = _fpRegisters.size();
     _fixedFPRegisterRanges.reserve(fpRegCount);
     for (int fpReg = 0; fpReg < fpRegCount; ++fpReg) {
-        LifeTimeInterval *lti = cloneFixedInterval(fpReg, true, ltiWithCalls);
-        _fixedFPRegisterRanges.append(lti);
-        if (lti->isValid())
-            _active.append(lti);
+        if (_fpRegisters.at(fpReg)->isCallerSaved()) {
+            LifeTimeInterval *lti = cloneFixedInterval(fpReg, true, ltiWithCalls);
+            _fixedFPRegisterRanges.append(lti);
+            if (lti->isValid())
+                _active.append(lti);
+        }
     }
 }
 
diff --git a/src/qml/jit/qv4targetplatform_p.h b/src/qml/jit/qv4targetplatform_p.h
index 1f08a0df2b..95db247d0f 100644
--- a/src/qml/jit/qv4targetplatform_p.h
+++ b/src/qml/jit/qv4targetplatform_p.h
@@ -61,6 +61,13 @@ namespace JIT {
 //    restore ebx (which holds the GOT ptr) before a call
 //  - All (supported) ARM platforms, where the only variety is the platform specific usage of r9,
 //    and the frame-pointer in Thumb/Thumb2 v.s. ARM mode.
+//
+// Specific handling of ebx when it holds the GOT:
+// In this case we can use it, but it needs to be restored when doing a call. So, the handling is as
+// follows: it is marked as caller saved, meaning the value in it won't survive a call. When
+// calculating the list of callee saved registers in getCalleeSavedRegisters (which is used to
+// generate push/pop instructions in the prelude/postlude), we add ebx too. Then when synthesizing
+// a call, we add a load it right before emitting the call instruction.
 class TargetPlatform
 {
 public:
@@ -81,7 +88,7 @@ public:
         typedef RegisterInfo RI;
         return RegisterInformation()
                 << RI(JSC::X86Registers::edx, QStringLiteral("edx"),   RI::RegularRegister,       RI::CallerSaved, RI::RegAlloc)
-                << RI(JSC::X86Registers::ebx, QStringLiteral("ebx"),   RI::RegularRegister,       RI::CalleeSaved, RI::RegAlloc)
+                << RI(JSC::X86Registers::ebx, QStringLiteral("ebx"),   RI::RegularRegister,       RI::CallerSaved, RI::RegAlloc)
                 << RI(JSC::X86Registers::edi, QStringLiteral("edi"),   RI::RegularRegister,       RI::CalleeSaved, RI::Predefined)
                 << RI(JSC::X86Registers::esi, QStringLiteral("esi"),   RI::RegularRegister,       RI::CalleeSaved, RI::Predefined)
                 << RI(JSC::X86Registers::xmm2, QStringLiteral("xmm2"), RI::FloatingPointRegister, RI::CallerSaved, RI::RegAlloc)
@@ -351,6 +358,12 @@ public: // utility functions
         static RegisterInformation regs;
         if (regs.isEmpty()) {
             foreach (const RegisterInfo &info, getRegisterInfo()) {
+#if defined(RESTORE_EBX_ON_CALL)
+                if (info.reg<JSC::X86Registers::RegisterID>() == JSC::X86Registers::ebx) {
+                    regs.append(info);
+                    continue;
+                }
+#endif // RESTORE_EBX_ON_CALL
                 if (info.isCalleeSaved())
                     regs.append(info);
             }