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add some notes about strlen craziness in eon.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61917 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2009-01-08 06:52:57 +00:00
parent 538fbb5086
commit ffb08f569b
1 changed files with 57 additions and 0 deletions
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lib/Target/README.txt
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@ -1495,3 +1495,60 @@ codegen.
Those should be turned into a switch.
//===---------------------------------------------------------------------===//
252.eon contains this interesting code:
%3072 = getelementptr [100 x i8]* %tempString, i32 0, i32 0
%3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind
%strlen = call i32 @strlen(i8* %3072) ; uses = 1
%endptr = getelementptr [100 x i8]* %tempString, i32 0, i32 %strlen
call void @llvm.memcpy.i32(i8* %endptr,
i8* getelementptr ([5 x i8]* @"\01LC42", i32 0, i32 0), i32 5, i32 1)
%3074 = call i32 @strlen(i8* %endptr) nounwind readonly
This is interesting for a couple reasons. First, in this:
%3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind
%strlen = call i32 @strlen(i8* %3072)
The strlen could be replaced with: %strlen = sub %3072, %3073, because the
strcpy call returns a pointer to the end of the string. Based on that, the
endptr GEP just becomes equal to 3073, which eliminates a strlen call and GEP.
Second, the memcpy+strlen strlen can be replaced with:
%3074 = call i32 @strlen([5 x i8]* @"\01LC42") nounwind readonly
Because the destination was just copied into the specified memory buffer. This,
in turn, can be constant folded to "4".
In other code, it contains:
%endptr6978 = bitcast i8* %endptr69 to i32*
store i32 7107374, i32* %endptr6978, align 1
%3167 = call i32 @strlen(i8* %endptr69) nounwind readonly
Which could also be constant folded. Whatever is producing this should probably
be fixed to leave this as a memcpy from a string.
Further, eon also has an interesting partially redundant strlen call:
bb8: ; preds = %_ZN18eonImageCalculatorC1Ev.exit
%682 = getelementptr i8** %argv, i32 6 ; <i8**> [#uses=2]
%683 = load i8** %682, align 4 ; <i8*> [#uses=4]
%684 = load i8* %683, align 1 ; <i8> [#uses=1]
%685 = icmp eq i8 %684, 0 ; <i1> [#uses=1]
br i1 %685, label %bb10, label %bb9
bb9: ; preds = %bb8
%686 = call i32 @strlen(i8* %683) nounwind readonly
%687 = icmp ugt i32 %686, 254 ; <i1> [#uses=1]
br i1 %687, label %bb10, label %bb11
bb10: ; preds = %bb9, %bb8
%688 = call i32 @strlen(i8* %683) nounwind readonly
This could be eliminated by doing the strlen once in bb8, saving code size and
improving perf on the bb8->9->10 path.
//===---------------------------------------------------------------------===//