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Query regarding RTL Template in i386.md
- From: Piyush Porwal <porwalpiyush at cse dot iitb dot ac dot in>
- To: gcc-help at gcc dot gnu dot org
- Date: Thu, 29 Sep 2005 11:40:16 +0530
- Subject: Query regarding RTL Template in i386.md
Hello everyone,
I would like to request your help in a specific matter which
relates to i386.md (GCC-4.0).
I find it difficult to understand the motivation to use
two or more 'set' constructs in an RTL template. Here is an example
from i386.md (GCC-4.0) file (only RTL Template is shown):
----------------------------------------------------------------------
(define_insn "*adddi_2_rex64"
[(set (reg FLAGS_REG)
(compare
(plus:DI (match_operand:DI 1 "nonimmediate_operand" "%0,0")
(match_operand:DI 2 "x86_64_general_operand" "rme,re"))
(const_int 0)))
(set (match_operand:DI 0 "nonimmediate_operand" "=r,rm")
(plus:DI (match_dup 1) (match_dup 2)))]
----------------------------------------------------------------------
As visible in the template, it contains two 'set' operators. A few
questions that come to mind are:
- the use of multiple 'set' constructs: in particular, what
aspect(s) of the instruction set, or the target machine, guides us
to choose such a construction,
- how does one 'evaluate' the benefits if other patterns with one
'set' are possible, (this particularly pertains to the decision of
where to place the insn pattern in the overall machine
description, since GCC uses a 'first hit' approach).
An example C code fragment would be extremely useful, if possible.
I deeply appreciate the time and effort you would be spending in
replying. I will be very thankful if you can provide me with your expert
guidance and thoughts.
-regards
__________________________________________
Piyush Porwal
CSE - M.Tech.
Indian Institute of Technology, Bombay
India
http://www.cse.iitb.ac.in/~porwalpiyush
__________________________________________