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3d834bf2c3
this transformation used to take a loop like this:
int Array[1000];
void test(int X) {
int i;
for (i = 0; i < 1000; ++i)
Array[i] += X;
}
Compiled to LLVM is:
no_exit: ; preds = %entry, %no_exit
%indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ] ; <uint> [#uses=2]
%tmp.4 = getelementptr [1000 x int]* %Array, int 0, uint %indvar ; <int*> [#uses=2]
%tmp.7 = load int* %tmp.4 ; <int> [#uses=1]
%tmp.9 = add int %tmp.7, %X ; <int> [#uses=1]
store int %tmp.9, int* %tmp.4
*** %indvar.next = add uint %indvar, 1 ; <uint> [#uses=2]
*** %exitcond = seteq uint %indvar.next, 1000 ; <bool> [#uses=1]
br bool %exitcond, label %return, label %no_exit
and turn it into a loop like this:
no_exit: ; preds = %entry, %no_exit
%indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ] ; <uint> [#uses=3]
%tmp.4 = getelementptr [1000 x int]* %Array, int 0, uint %indvar ; <int*> [#uses=2]
%tmp.7 = load int* %tmp.4 ; <int> [#uses=1]
%tmp.9 = add int %tmp.7, %X ; <int> [#uses=1]
store int %tmp.9, int* %tmp.4
*** %indvar.next = add uint %indvar, 1 ; <uint> [#uses=1]
*** %exitcond = seteq uint %indvar, 999 ; <bool> [#uses=1]
br bool %exitcond, label %return, label %no_exit
Note that indvar.next and indvar can no longer be coallesced. In machine
code terms, this patch changes this code:
.LBBtest_1: # no_exit
mov %EDX, OFFSET Array
mov %ESI, %EAX
add %ESI, DWORD PTR [%EDX + 4*%ECX]
mov %EDX, OFFSET Array
mov DWORD PTR [%EDX + 4*%ECX], %ESI
mov %EDX, %ECX
inc %EDX
cmp %ECX, 999
mov %ECX, %EDX
jne .LBBtest_1 # no_exit
into this:
.LBBtest_1: # no_exit
mov %EDX, OFFSET Array
mov %ESI, %EAX
add %ESI, DWORD PTR [%EDX + 4*%ECX]
mov %EDX, OFFSET Array
mov DWORD PTR [%EDX + 4*%ECX], %ESI
inc %ECX
cmp %ECX, 1000
jne .LBBtest_1 # no_exit
We need better instruction selection to get this:
.LBBtest_1: # no_exit
add DWORD PTR [Array + 4*%ECX], EAX
inc %ECX
cmp %ECX, 1000
jne .LBBtest_1 # no_exit
... but at least there is less register juggling
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16473 91177308-0d34-0410-b5e6-96231b3b80d8
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