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Now that codegen prepare isn't defeating me, I can finally fix what I set

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out to do! :)

This fixes a problem where LSR would insert a bunch of code into each MBB
that uses a particular subexpression (e.g. IV+base+C).  The problem is that
this code cannot be CSE'd back together if inserted into different blocks.

This patch changes LSR to attempt to insert a single copy of this code and
share it, allowing codegenprepare to duplicate the code if it can be sunk
into various addressing modes.  On CodeGen/ARM/lsr-code-insertion.ll,
for example, this gives us code like:

        add r8, r0, r5
        str r6, [r8, #+4]
..
        ble LBB1_4      @cond_next
LBB1_3: @cond_true
        str r10, [r8, #+4]
LBB1_4: @cond_next
...
LBB1_5: @cond_true55
        ldr r6, LCPI1_1
        str r6, [r8, #+4]

instead of:

        add r10, r0, r6
        str r8, [r10, #+4]
...
        ble LBB1_4      @cond_next
LBB1_3: @cond_true
        add r8, r0, r6
        str r10, [r8, #+4]
LBB1_4: @cond_next
...
LBB1_5: @cond_true55
        add r8, r0, r6
        ldr r10, LCPI1_1
        str r10, [r8, #+4]

Besides being smaller and more efficient, this makes it immediately
obvious that it is profitable to predicate LBB1_3 now :)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35972 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2007-04-13 20:42:26 +00:00
parent 3bd767b91d
commit c5494af8a9
1 changed files with 19 additions and 1 deletions
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20
lib/Transforms/Scalar/LoopStrengthReduce.cpp
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@ -568,7 +568,25 @@ void BasedUser::RewriteInstructionToUseNewBase(const SCEVHandle &NewBase,
SCEVExpander &Rewriter,
Loop *L, Pass *P) {
if (!isa<PHINode>(Inst)) {
Value *NewVal = InsertCodeForBaseAtPosition(NewBase, Rewriter, Inst, L);
// By default, insert code at the user instruction.
BasicBlock::iterator InsertPt = Inst;
// However, if the Operand is itself an instruction, the (potentially
// complex) inserted code may be shared by many users. Because of this, we
// want to emit code for the computation of the operand right before its old
// computation. This is usually safe, because we obviously used to use the
// computation when it was computed in its current block. However, in some
// cases (e.g. use of a post-incremented induction variable) the NewBase
// value will be pinned to live somewhere after the original computation.
// In this case, we have to back off.
if (!isUseOfPostIncrementedValue) {
if (Instruction *OpInst = dyn_cast<Instruction>(OperandValToReplace)) {
InsertPt = OpInst;
while (isa<PHINode>(InsertPt)) ++InsertPt;
}
}
Value *NewVal = InsertCodeForBaseAtPosition(NewBase, Rewriter, InsertPt, L);
// Replace the use of the operand Value with the new Phi we just created.
Inst->replaceUsesOfWith(OperandValToReplace, NewVal);
DOUT << " CHANGED: IMM =" << *Imm << " Inst = " << *Inst;