Hi,
I have some troubles to figure out why clang is removing my label name. As the
simple example below shows the label "MLB" is removed and replaced by
an integer as an "unnamed value".
Plus, I can see that an unconditional jump is added, as the label seems to be
interpreted as the beginning of a BasicBlock. I am compiling without any
optimizations (for now), what leave the produced code slower than the input one.
Anyway, the important thinks to me is to keep label name, but I can't figure
out where in the code this transformation is done. My guess is, it's done by
Clang while parsing, or just after. Could someone gives me some clue about the
class/method where this "feature" is ?
Input:
int main() {
int i=0;
MLB:
return i;
}
Result:
define i32 @main() #0 {
%1 = alloca i32, align 4
%i = alloca i32, align 4
store i32 0, i32* %1
store i32 0, i32* %i, align 4
br label %2
; <label>:2 ; preds = %0
%3 = load i32* %i, align 4
ret i32 %3
}
---------------------------------------------------
% clang --version
clang version 3.4 (branches/release_34)
Target: x86_64-unknown-linux-gnu
Thread model: posix
% clang -O0 -emit-llvm -mllvm -debug-pass=Structure -S -c -o /dev/stdout main.c
Thanks for you help
Benjamin Rouxel
Hi, So finally I found where the label was evicted. In case it's useful for someone, i needed to add: "Dest.getBlock()->setName(D->getName());" in CGStmt.cpp:360. About the creation of the BasicBlock, It's in the same method, but as expected it's removed with optimization (O1,O2,O3) and the label name is kept trough CFG optimizations. Cheers, Benjamin Rouxel Le Fri, 6 Jun 2014 10:23:21 +0200, Rouxel Benjamin <benjamin.rouxel at inria.fr> a écrit :> Hi, > > I have some troubles to figure out why clang is removing my label name. As the simple example below shows the label "MLB" is removed and replaced by an integer as an "unnamed value". > Plus, I can see that an unconditional jump is added, as the label seems to be interpreted as the beginning of a BasicBlock. I am compiling without any optimizations (for now), what leave the produced code slower than the input one. > > Anyway, the important thinks to me is to keep label name, but I can't figure out where in the code this transformation is done. My guess is, it's done by Clang while parsing, or just after. Could someone gives me some clue about the class/method where this "feature" is ? > > Input: > int main() { > int i=0; > MLB: > return i; > } > > Result: > define i32 @main() #0 { > %1 = alloca i32, align 4 > %i = alloca i32, align 4 > store i32 0, i32* %1 > store i32 0, i32* %i, align 4 > br label %2 > > ; <label>:2 ; preds = %0 > %3 = load i32* %i, align 4 > ret i32 %3 > } > > --------------------------------------------------- > % clang --version > clang version 3.4 (branches/release_34) > Target: x86_64-unknown-linux-gnu > Thread model: posix > > % clang -O0 -emit-llvm -mllvm -debug-pass=Structure -S -c -o /dev/stdout main.c > > Thanks for you help > Benjamin Rouxel
In LLVM, every block must explicitly specify how control flows out of that block, the order that blocks are printed to text / bitcode has no effect on the control flow whatsoever. The label forces the creation of a second block, so the first one must end with a branch to it. Since there's only one code path, LLVM will later merge the two blocks together. On Fri, Jun 6, 2014 at 11:32 PM, Rouxel Benjamin <benjamin.rouxel at inria.fr> wrote:> Hi, > > So finally I found where the label was evicted. In case it's useful for > someone, i needed to add: "Dest.getBlock()->setName(D->getName());" in > CGStmt.cpp:360. > > About the creation of the BasicBlock, It's in the same method, but as > expected it's removed with optimization (O1,O2,O3) and the label name is > kept trough CFG optimizations. > > Cheers, > Benjamin Rouxel > > Le Fri, 6 Jun 2014 10:23:21 +0200, > Rouxel Benjamin <benjamin.rouxel at inria.fr> a écrit : > > > Hi, > > > > I have some troubles to figure out why clang is removing my label name. > As the simple example below shows the label "MLB" is removed and replaced > by an integer as an "unnamed value". > > Plus, I can see that an unconditional jump is added, as the label seems > to be interpreted as the beginning of a BasicBlock. I am compiling without > any optimizations (for now), what leave the produced code slower than the > input one. > > > > Anyway, the important thinks to me is to keep label name, but I can't > figure out where in the code this transformation is done. My guess is, it's > done by Clang while parsing, or just after. Could someone gives me some > clue about the class/method where this "feature" is ? > > > > Input: > > int main() { > > int i=0; > > MLB: > > return i; > > } > > > > Result: > > define i32 @main() #0 { > > %1 = alloca i32, align 4 > > %i = alloca i32, align 4 > > store i32 0, i32* %1 > > store i32 0, i32* %i, align 4 > > br label %2 > > > > ; <label>:2 ; preds = %0 > > %3 = load i32* %i, align 4 > > ret i32 %3 > > } > > > > --------------------------------------------------- > > % clang --version > > clang version 3.4 (branches/release_34) > > Target: x86_64-unknown-linux-gnu > > Thread model: posix > > > > % clang -O0 -emit-llvm -mllvm -debug-pass=Structure -S -c -o /dev/stdout > main.c > > > > Thanks for you help > > Benjamin Rouxel > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140607/a0137653/attachment.html>