[Webkit-unassigned] [Bug 251421] New: Remove unnecessary add when loading a JSCConfig field.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=251421

            Bug ID: 251421
           Summary: Remove unnecessary add when loading a JSCConfig field.
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mark.lam at apple.com

Currently, to load a JSCConfig field, our LLInt asm does something like this:
```
    leap JSCConfig + constexpr JSC::offsetOfJSCConfigGateMap + (constexpr Gate::%opcodeName%) * PtrSize, ws1
    jmp [ws1], NativeToJITGatePtrTag # JSEntrySlowPathPtrTag
```
... and generates this:
```
                  #if OS(DARWIN)
".loc 1 1\n"          "Ljsc_llint_loh_adrp_1508: \n"       // LowLevelInterpreter.asm:1
                      "adrp x10, " LOCAL_REFERENCE(g_config) "@GOTPAGE \n"
                      "Ljsc_llint_loh_ldr_1508: \n"
                      "ldr x10, [x10, " LOCAL_REFERENCE(g_config) "@GOTPAGEOFF] \n"
                  #elif OS(LINUX)
                      ...
                  #endif
".loc 1 1\n"          "add x10, x10, #3592 \n"   // <---- this add can be applied as an offset to the ldr below.
".loc 6 1034\n"       "movz x13, #57366 \n"                // WebAssembly.asm:1034
                      "ldr x17, [x10] \n"
                      "brab x17, x13 \n"
```

This patch re-arranges the LLInt assembly to look like this instead:
```
    leap _g_config, ws1
    jmp JSCConfigGateMapOffset + (constexpr Gate::%opcodeName%) * PtrSize[ws1], NativeToJITGatePtrTag # JSEntrySlowPathPtrTag
```
... resulting in the removal of the unnecessary add instruction:
```
                  #if OS(DARWIN)
".loc 1 1\n"          "Ljsc_llint_loh_adrp_1508: \n"       // LowLevelInterpreter.asm:1
                      "adrp x10, " LOCAL_REFERENCE(g_config) "@GOTPAGE \n"
                      "Ljsc_llint_loh_ldr_1508: \n"
                      "ldr x10, [x10, " LOCAL_REFERENCE(g_config) "@GOTPAGEOFF] \n"
                  #elif OS(LINUX)
                      ...
                  #endif
".loc 6 1034\n"       "movz x13, #57366 \n"                // WebAssembly.asm:1034
                      "ldr x17, [x10, #3592] \n"
                      "brab x17, x13 \n"
```

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