Bug 241856 – Change offlineasm to emit more efficient LLInt code.

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Mark Lam]

1. Ruby treats numeric 0 as truthy.  However, there's a test in arm64LowerMalformedLoadStoreAddresses which assumes a value of 0 would be false.  As a result, we see offlineasm emit inefficient LLInt code like this:
    ".loc 3 821\n"        "movz x16, #0 \n"                    // LowLevelInterpreter64.asm:821
                          "add x13, x3, x16 \n"
                          "ldr x0, [x13] \n"

  ...  instead of this:
    ".loc 3 821\n"        "ldr x0, [x3] \n"                    // LowLevelInterpreter64.asm:821

   This patch fixes this.

2. offlineasm's emitARM64MoveImmediate chooses to use `movn` instead of `movz` based on whether a 64-bit value is negative or not.  Instead, it should be making that decision based on the number of halfwords (16-bits) in the value that is 0xffff vs 0.  As a result, offlineasm emits code like this:
    ".loc 1 1638\n"       "movn x27, #1, lsl #48 \n"           // LowLevelInterpreter.asm:1638
                          "movk x27, #0, lsl #32 \n"
                          "movk x27, #0, lsl #16 \n"
                          "movk x27, #0 \n"

  ...  instead of this:
    ".loc 1 1638\n"       "movz x27, #65534, lsl #48 \n"       // LowLevelInterpreter.asm:1638

   This patch fixes this.

3. offlineasm is trivially assuming the range of immediate offsets for ldr/str instructions is [-255..4095].  However, that's only the range for byte sized load-stores.  For 32-bit, the range is [-255..16380].  For 64-bit, the range is [-255..32760].  As a result, offlineasm emits code like this:
    ".loc 1 633\n"        "movn x16, #16383 \n"                // LowLevelInterpreter.asm:633
    ".loc 1 1518\n"       "and x3, x3, x16 \n"                 // LowLevelInterpreter.asm:1518
    ".loc 1 1519\n"       "movz x16, #16088 \n"                // LowLevelInterpreter.asm:1519
                          "add x17, x3, x16 \n"
                          "ldr x3, [x17] \n"

  ...  instead of this:
    ".loc 1 633\n"        "movn x17, #16383 \n"                // LowLevelInterpreter.asm:633
    ".loc 1 1518\n"       "and x3, x3, x17 \n"                 // LowLevelInterpreter.asm:1518
    ".loc 1 1519\n"       "ldr x3, [x3, #16088] \n"            // LowLevelInterpreter.asm:1519

   This patch fixes this for 64-bit and 32-bit load-stores.  16-bit load-stores also has a wider range, but for now, it will continue to use the conservative range.
   This patch also introduces an `isMalformedArm64LoadAStoreAddress` so that this range check can be done consistently in all the places that checks for it.

4. offlineasm is eagerly emitting no-op arguments in instructions, e.g. "lsl #0", and adding 0.  As a result, offlineasm emits code like this:
    ".loc 3 220\n"        "movz x13, #51168, lsl #0 \n"        // LowLevelInterpreter64.asm:220
                          "add x17, x1, x13, lsl #0 \n"
                          "ldr w4, [x17, #0] \n"

  ...  instead of this:
    ".loc 3 220\n"        "movz x13, #51168 \n"                // LowLevelInterpreter64.asm:220
                          "add x17, x1, x13 \n"
                          "ldr w4, [x17] \n"

   This unnecessary arguments are actually very common throughout the emitted LLIntAssembly.h.

   This patch removes these unnecessary arguments, which makes the emitted LLInt code more human readable due to less clutter.

[Mark Lam]

PR: https://github.com/WebKit/WebKit/pull/1686

[Mark Lam]

Created attachment 460419 [details]
LLIntAssembly-before.h

[Mark Lam]

Created attachment 460420 [details]
LLIntAssembly-before-wo-unnecessary-lsl.h

[Mark Lam]

Created attachment 460421 [details]
LLIntAssembly-after.h

[EWS]

Committed r295766 (251771@main): <https://commits.webkit.org/251771@main>

Reviewed commits have been landed. Closing PR #1686 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/95743853>