Closed
Bug 581902
Opened 14 years ago
Closed 11 years ago
JM: Eliminate Redundant Comparison in jsop_binary()
Categories
(Core :: JavaScript Engine, defect)
Core
JavaScript Engine
Tracking
()
RESOLVED
WONTFIX
People
(Reporter: sstangl, Unassigned)
References
Details
x = "cats"
x+5
(breakpoint hit at top of jsop_binary at runtime):
=> 0x7ffff1d2b1fb: mov 0x90(%rbx),%r14
0x7ffff1d2b202: movabs $0xffff800000000000,%r11
0x7ffff1d2b20c: and %r11,%r14
0x7ffff1d2b20f: mov 0x90(%rbx),%r13
0x7ffff1d2b216: movabs $0x7fffffffffff,%r11
0x7ffff1d2b220: and %r11,%r13
0x7ffff1d2b223: mov %r13,%r12
0x7ffff1d2b226: movabs $0xfff8800000000000,%r11
0x7ffff1d2b230: cmp %r11,%r14
0x7ffff1d2b233: jne 0x7ffff1d2b2e5
Hits that, jumps to:
=> 0x7ffff1d2b2e5: movabs $0xfff8000000000000,%r11
0x7ffff1d2b2ef: cmp %r11,%r14
0x7ffff1d2b2f2: jae 0x7ffff1d2b372
The "movabs" and "cmp" have no effect and can be safely eliminated. On x86/ARM, this results in the elimination of a single 'cmp' per double/slow path.
We need to first verify that ARM does not reset processor flags on a branch. (Though it would be very silly if that were the case.)
Comment 1•14 years ago
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(In reply to comment #0)
> We need to first verify that ARM does not reset processor flags on a branch.
> (Though it would be very silly if that were the case.)
This is fine on ARM. Branches don't touch flags.
Updated•14 years ago
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Blocks: JaegerPunyWins
Comment 2•11 years ago
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JM was removed.
Status: NEW → RESOLVED
Closed: 11 years ago
Resolution: --- → WONTFIX
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Description
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