[llvm] r251492 - [IndVarSimplify] Rewrite loop exit values with their initial values from loop preheader
Sanjoy Das via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 27 22:08:01 PDT 2015
Looks like this broke some bots
http://lab.llvm.org:8011/builders/clang-cmake-mips/builds/10258/steps/ninja%20check%201/logs/stdio
Sent from a mobile device, please excuse typos.
On Oct 27, 2015 21:47, "Chen Li via llvm-commits" <
llvm-commits at lists.llvm.org> wrote:
> Author: chenli
> Date: Tue Oct 27 23:45:47 2015
> New Revision: 251492
>
> URL: http://llvm.org/viewvc/llvm-project?rev=251492&view=rev
> Log:
> [IndVarSimplify] Rewrite loop exit values with their initial values from
> loop preheader
>
> Summary:
> This patch adds support to check if a loop has loop invariant conditions
> which lead to loop exits. If so, we know that if the exit path is taken, it
> is at the first loop iteration. If there is an induction variable used in
> that exit path whose value has not been updated, it will keep its initial
> value passing from loop preheader. We can therefore rewrite the exit value
> with
> its initial value. This will help remove phis created by LCSSA and enable
> other optimizations like loop unswitch.
>
>
> Reviewers: sanjoy
>
> Subscribers: llvm-commits
>
> Differential Revision: http://reviews.llvm.org/D13974
>
> Added:
> llvm/trunk/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
> Modified:
> llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
> llvm/trunk/test/Transforms/LoopUnroll/scevunroll.ll
>
> Modified: llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp?rev=251492&r1=251491&r2=251492&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp Tue Oct 27
> 23:45:47 2015
> @@ -132,6 +132,7 @@ private:
>
> bool canLoopBeDeleted(Loop *L, SmallVector<RewritePhi, 8>
> &RewritePhiSet);
> void rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter);
> + void rewriteFirstIterationLoopExitValues(Loop *L);
>
> Value *linearFunctionTestReplace(Loop *L, const SCEV
> *BackedgeTakenCount,
> PHINode *IndVar, SCEVExpander
> &Rewriter);
> @@ -700,6 +701,71 @@ void IndVarSimplify::rewriteLoopExitValu
> Rewriter.clearInsertPoint();
> }
>
>
> +//===---------------------------------------------------------------------===//
> +// rewriteFirstIterationLoopExitValues: Rewrite loop exit values if we
> know
> +// they will exit at the first iteration.
>
> +//===---------------------------------------------------------------------===//
> +
> +/// Check to see if this loop has loop invariant conditions which lead to
> loop
> +/// exits. If so, we know that if the exit path is taken, it is at the
> first
> +/// loop iteration. This lets us predict exit values of PHI nodes that
> live in
> +/// loop header.
> +void IndVarSimplify::rewriteFirstIterationLoopExitValues(Loop *L) {
> + // Verify the input to the pass is already in LCSSA form.
> + assert(L->isLCSSAForm(*DT));
> +
> + SmallVector<BasicBlock *, 8> ExitBlocks;
> + L->getUniqueExitBlocks(ExitBlocks);
> +
> + for (auto *ExitBB : ExitBlocks) {
> + BasicBlock::iterator begin = ExitBB->begin();
> + // If there are no more PHI nodes in this exit block, then no more
> + // values defined inside the loop are used on this path.
> + while (auto *PN = dyn_cast<PHINode>(begin++)) {
> + for (unsigned IncomingValIdx = 0, e = PN->getNumIncomingValues();
> + IncomingValIdx != e; ++IncomingValIdx) {
> + auto *IncomingBB = PN->getIncomingBlock(IncomingValIdx);
> + if (!L->contains(IncomingBB))
> + continue;
> +
> + // Get condition that leads to the exit path.
> + auto *TermInst = IncomingBB->getTerminator();
> +
> + Value *Cond = nullptr;
> + if (auto *BI = dyn_cast<BranchInst>(TermInst)) {
> + // Must be a conditional branch, otherwise the block
> + // should not be in the loop.
> + Cond = BI->getCondition();
> + } else if (auto *SI = dyn_cast<SwitchInst>(TermInst))
> + Cond = SI->getCondition();
> +
> + // All non-instructions are loop-invariant.
> + if (isa<Instruction>(Cond) && !L->isLoopInvariant(Cond))
> + continue;
> +
> + auto *ExitVal =
> + dyn_cast<PHINode>(PN->getIncomingValue(IncomingValIdx));
> +
> + // Only deal with PHIs.
> + if (!ExitVal)
> + continue;
> +
> + // If ExitVal is a PHI on the loop header, then we know its
> + // value along this exit because the exit can only be taken
> + // on the first iteration.
> + auto *LoopPreheader = L->getLoopPreheader();
> + assert(LoopPreheader && "Invalid loop");
> + if (ExitVal->getBasicBlockIndex(LoopPreheader) != -1) {
> + assert(ExitVal->getParent() == L->getHeader() &&
> + "ExitVal must be in loop header");
> + PN->setIncomingValue(IncomingValIdx,
> + ExitVal->getIncomingValueForBlock(LoopPreheader));
> + }
> + }
> + }
> + }
> +}
> +
> /// Check whether it is possible to delete the loop after rewriting exit
> /// value. If it is possible, ignore ReplaceExitValue and do rewriting
> /// aggressively.
> @@ -2173,6 +2239,11 @@ bool IndVarSimplify::runOnLoop(Loop *L,
> // loop may be sunk below the loop to reduce register pressure.
> sinkUnusedInvariants(L);
>
> + // rewriteFirstIterationLoopExitValues does not rely on the computation
> of
> + // trip count and therefore can further simplify exit values in
> addition to
> + // rewriteLoopExitValues.
> + rewriteFirstIterationLoopExitValues(L);
> +
> // Clean up dead instructions.
> Changed |= DeleteDeadPHIs(L->getHeader(), TLI);
> // Check a post-condition.
>
> Added: llvm/trunk/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll?rev=251492&view=auto
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
> (added)
> +++ llvm/trunk/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
> Tue Oct 27 23:45:47 2015
> @@ -0,0 +1,75 @@
> +; RUN: opt -indvars -instcombine -S < %s | FileCheck %s
> +
> +;; Test that loop's exit value is rewritten to its initial
> +;; value from loop preheader
> +define i32 @test1(i32* %var) {
> +; CHECK-LABEL: @test1
> +entry:
> + %cond = icmp eq i32* %var, null
> + br label %header
> +
> +header:
> + %phi_indvar = phi i32 [0, %entry], [%indvar, %loop]
> + br i1 %cond, label %loop, label %exit
> +
> +loop:
> + %indvar = add i32 %phi_indvar, 1
> + br label %header
> +
> +exit:
> +; CHECK: ret i32 0
> + ret i32 %phi_indvar
> +}
> +
> +
> +;; Test that inner loop's exit value is first rewritten to outer
> +;; loop's induction variable, and then further rewritten to a
> +;; constant when process outer loop.
> +define i32 @test2(i32* %var1, i32* %var2) {
> +; CHECK-LABEL: @test2
> +entry:
> + %cond1 = icmp eq i32* %var1, null
> + %cond2 = icmp eq i32* %var2, null
> + br label %outer_header
> +
> +outer_header:
> + %phi_outer = phi i32 [0, %entry], [%indvar_outer, %inner_exit]
> + br label %inner_header
> +
> +inner_header:
> + %phi_inner = phi i32 [%phi_outer, %outer_header], [%indvar_inner, %loop]
> + br i1 %cond1, label %loop, label %exit
> +
> +loop:
> + %indvar_inner = add i32 %phi_inner, 1
> + br i1 %cond2, label %inner_header, label %inner_exit
> +
> +inner_exit:
> + %indvar_outer = add i32 %phi_outer, 1
> + br label %outer_header
> +
> +exit:
> +;; %phi_inner is first rewritten to %phi_outer
> +;; and then %phi_outer is rewritten to 0
> + %ret_val = add i32 %phi_inner, %phi_outer
> +; CHECK: ret i32 0
> + ret i32 %ret_val
> +}
> +
> +;; Test that we can not rewrite loop exit value if it's not
> +;; a phi node (%indvar is an add instruction in this test).
> +define i32 @test3(i32* %var) {
> +; CHECK-LABEL: @test3
> +entry:
> + %cond = icmp eq i32* %var, null
> + br label %header
> +
> +header:
> + %phi_indvar = phi i32 [0, %entry], [%indvar, %header]
> + %indvar = add i32 %phi_indvar, 1
> + br i1 %cond, label %header, label %exit
> +
> +exit:
> +; CHECK: ret i32 %indvar
> + ret i32 %indvar
> +}
> \ No newline at end of file
>
> Modified: llvm/trunk/test/Transforms/LoopUnroll/scevunroll.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/scevunroll.ll?rev=251492&r1=251491&r2=251492&view=diff
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/LoopUnroll/scevunroll.ll (original)
> +++ llvm/trunk/test/Transforms/LoopUnroll/scevunroll.ll Tue Oct 27
> 23:45:47 2015
> @@ -184,7 +184,7 @@ for.body87:
> ; CHECK: for.body:
> ; CHECK: %b.03 = phi i32 [ 0, %entry ], [ %add, %for.cond ]
> ; CHECK: return:
> -; CHECK: %b.03.lcssa = phi i32 [ %b.03, %for.body ], [ %b.03, %for.cond ]
> +; CHECK: %b.03.lcssa = phi i32 [ %b.03, %for.body ], [ 0, %for.cond ]
> define void @nsw_latch(i32* %a) nounwind {
> entry:
> br label %for.body
>
>
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