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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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.. | ||
Hello | ||
Instrumentation | ||
IPO | ||
Scalar | ||
Utils | ||
ExprTypeConvert.cpp | ||
LevelRaise.cpp | ||
Makefile | ||
TransformInternals.cpp | ||
TransformInternals.h |