https://sourceware.org/bugzilla/show_bug.cgi?id=22871
Bug ID: 22871
Summary: Encode instructions of 64-bit registers without the
REX_W bit
Product: binutils
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: hjl.tools at gmail dot com
Target Milestone: ---
Target: x86-64
When the upper 32 bits of destination registers of instructions are known
to be zero, we can encode them without the REX_W bit. For example,
andq $imm31, %r64
xorq %r64, %r64
can be encoded as:
andl $imm31, %r32
xorl %r32, %r32
To make assembler more predictable, this optimization should be done only
when -O is passed to assembler.
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