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Chris Lattner a061859ccc Teach instcombine to fold compares of loads from constant 
arrays with variable indices into a comparison of the index
with a constant.  The most common occurrence of this that
I see by far is stuff like:

if ("foobar"[i] == '\0') ...

which we compile into: if (i == 6), saving a load and 
materialization of the global address.  This also exposes 
loop trip count information to later passes in many cases.

This triggers hundreds of times in xalancbmk, which is where I first
noticed it, but it also triggers in many other apps.  Here are a few 
interesting ones from various apps:

@must_be_connected_without = internal constant [8 x i8*] [i8* getelementptr inbounds ([3 x i8]* @.str64320, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str27283, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8]* @.str71327, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8]* @.str72328, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str18274, i64 0, i64 0), i8* getelementptr inbounds ([6 x i8]* @.str11267, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str32288, i64 0, i64 0), i8* null], align 32 ; <[8 x i8*]*> [#uses=2]
  %scevgep.i = getelementptr [8 x i8*]* @must_be_connected_without, i64 0, i64 %indvar.i ; <i8**> [#uses=1]
  %17 = load ...
  %18 = icmp eq i8* %17, null                     ; <i1> [#uses=1]
-> icmp eq i64 %indvar.i, 7 


@yytable1095 = internal constant [84 x i8] c"\12\01(\05\06\07\08\09\0A\0B\0C\0D\0E1\0F\10\11266\1D: \10\11,-,0\03'\10\11B6\04\17&\18\1945\05\06\07\08\09\0A\0B\0C\0D\0E\1E\0F\10\11*\1A\1B\1C$3+>#%;<IJ=ADFEGH9KL\00\00\00C", align 32 ; <[84 x i8]*> [#uses=2]
  %57 = getelementptr inbounds [84 x i8]* @yytable1095, i64 0, i64 %56 ; <i8*> [#uses=1]
   %mode.0.in = getelementptr inbounds [9 x i32]* @mb_mode_table, i64 0, i64 %.pn ; <i32*> [#uses=1]
load ...
   %64 = icmp eq i8 %58, 4                         ; <i1> [#uses=1]
-> icmp eq i64 %.pn, 35             ; <i1> [#uses=0]


@gsm_DLB = internal constant [4 x i16] [i16 6554, i16 16384, i16 26214, i16 32767]
%scevgep.i = getelementptr [4 x i16]* @gsm_DLB, i64 0, i64 %indvar.i ; <i16*> [#uses=1]
%425 = load %scevgep.i
%426 = icmp eq i16 %425, -32768                 ; <i1> [#uses=0]
-> false

llvm-svn: 92411
 		2010-01-02 08:12:04 +00:00
..
Analysis constant fold nasty constant expressions formed by llvm-gcc, 2010-01-02 01:22:23 +00:00
Archive Prune #includes from llvm/Linker.h and llvm/System/Path.h, 2009-08-23 22:45:37 +00:00
AsmParser remove some fixme's 2009-12-30 06:00:36 +00:00
Bitcode Remove derelict serialization code. 2009-12-31 23:40:17 +00:00
CodeGen Teach codegen to handle: 2010-01-02 00:00:03 +00:00
CompilerDriver Make Path use StringRef instead of std::string where possible. 2009-12-17 21:02:39 +00:00
ExecutionEngine Remove dead store and simplify code. 2009-12-28 01:53:00 +00:00
Linker rename "elements" of metadata to "operands". "Elements" are 2009-12-31 01:22:29 +00:00
MC Pass StringRef by value. 2009-11-06 10:58:06 +00:00
Support Document the edit-distance algorithm used in StringRef, switch it over 2009-12-31 04:24:34 +00:00
System Alternative fix to make sure that the extern declarations used by 2009-12-23 19:12:50 +00:00
Target Teach codegen to lower llvm.powi to an efficient (but not optimal) 2010-01-01 03:32:16 +00:00
Transforms Teach instcombine to fold compares of loads from constant 2010-01-02 08:12:04 +00:00
VMCore fix refactoro 2009-12-31 08:23:09 +00:00
Makefile remove llvm-db: it is completely broken and if anyone wants to do a debugger, 2009-10-05 02:29:51 +00:00