[Webkit-unassigned] [Bug 116306] New: MacroAssemblerARM should use xor to swap registers instead of move

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=116306

           Summary: MacroAssemblerARM should use xor to swap registers
                    instead of move
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rgabor at webkit.org
                CC: zherczeg at webkit.org, fpizlo at apple.com


On ARM Traditional and Thumb2 we are using a temporary register to swap two register value.
ARM Traditional using the r3 register for that purpose.

MacroAssemblerARM.h:540
     void swap(RegisterID reg1, RegisterID reg2)
     {
        move(reg1, ARMRegisters::S0);
        move(reg2, reg1);
        move(ARMRegisters::S0, reg2);
     }

That causing a problem because there is a case when we set up the function parameters in DFGCCallHelpers.h we put the third parameter to r3 then in setupTwoStubArgs() we swap r1 and r2 using r3 as temporary register. 

DFGCCallHelpers.h:460
        if (arg1 != GPRInfo::argumentGPR3 && arg2 != GPRInfo::argumentGPR3) {
            move(arg3, GPRInfo::argumentGPR3);
            setupTwoStubArgs<GPRInfo::argumentGPR1, GPRInfo::argumentGPR2>(arg1, arg2);
            return;
        }

This generates the following code which is not correct:
  => 0x40022a3c:  mov     r3, r4
  => 0x40022a40:  mov     r3, r1
  => 0x40022a44:  mov     r1, r2
  => 0x40022a48:  mov     r2, r3

ARM Thumb2 is using the r12 register so this bug doesn't affect it.

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