[Bug tree-optimization/31136] [4.2 Regression] FRE ignores bit-field truncation (C and C++ front-end don't produce bit-field truncation

[joseph at codesourcery dot com]

------- Comment #11 from joseph at codesourcery dot com  2007-03-23 13:41 -------
Subject: Re:  [4.2 Regression] FRE ignores
 bit-field truncation (C and C++ front-end don't produce bit-field truncation

On Fri, 23 Mar 2007, pinskia at gmail dot com wrote:

> No, STRIP_SIGN_NOPS is correct, just fold_unary is incorrect in its

That depends on an analysis of every caller of STRIP_SIGN_NOPS to work out 
what semantics they require and whether removing conversions changing the 
value is correct in that case.  Only then can you determine whether 
STRIP_SIGN_NOPS should have the present semantics and some subset of 
callers should be changed to work with those semantics, or whether the 
semantics of STRIP_SIGN_NOPS would better be changed.

On the whole I think that references to the mode in STRIP_NOPS and 
STRIP_SIGN_NOPS are rather doubtful - mode should not be of relevance at 
this level of tree optimizations - and mode is probably being used as a 
proxy for precision.  The general sequence of integer type conversions can 
be represented in the form "truncate to M bits, sign-extend to N bits and 
then zero-extend to the width of the outer type", maybe this should be 
represented somehow; then it would be defined exactly what such 
conversions can be removed by these macros.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31136