Custom Query (1030 matches)

r11754 is workaround for a bug in how the lisp assembler encodes memory operands that are just a displacement.

To elaborate on that commit message, there are two ways on 32-bit x86 to encode a memory operand that's just a displacement.

For example, take the instruction:

0x806e6db:	mov    %fs:0x84,%ecx

The lisp assembler encodes this as:

0x806e6db:	0x64	0x8b	0x0c	0x25	0x84	0x00	0x00	0x00

Note that the modrm byte of 0x0c (00 001 100) means that a sib byte follows.

This could also be encoded as this shorter sequence (and the Unix assembler does so):

0x806e6db:	0x64	0x8b	0x0d	0x84	0x00	0x00	0x00

The modrm byte of 0x0d (00 001 101) here means that the displacement follows.

The reason that the lisp assembler selects the longer encoding is because it targeted x86-64 first. On x86-64, the modrm byte in the shorter sequence is redefined to mean that the displacement is RIP-relative. Therefore, the longer sib byte encoding is used to specify just a displacement.

The lisp assembler needs to be persuaded to emit the shorter encoding for 32-bit x86. When it does, we can recompile and bump fasl versions, etc., and remove the workaround in pc_luser_xp().

Running the code in attached file, in which 10 threads cons hysterically, will pretty reliably elicit heap corruption.

From Matt Lamari, see ​http://clozure.com/pipermail/openmcl-devel/2011-May/012827.html.

For CASE forms with suitably small ranges of integer keys, the compiler could generate a jump table (or binary search, or a combination of the two) rather than the obvious linear search.