[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 22:56:38 PDT 2015


Looks like the bot is just building clang and running clang's test suite.

But I think I've spotted what the bug is:

         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();
         }


^^ you're missing an 'else' case here.

-- Sanjoy


Chen Li wrote:
> I will revert it for now and investigate it. Is there an easy way to rerun the failed test case?
>
> thanks,
> chen
>
>
>> On Oct 27, 2015, at 10:08 PM, Sanjoy Das <sanjoy at playingwithpointers.com <mailto:sanjoy at playingwithpointers.com>> wrote:
>>
>> Looks like this broke some bots
>> http://lab.llvm.org:8011/builders/clang-cmake-mips/builds/10258/steps/ninja%20check%201/logs/stdio
>> <http://lab.llvm.org:8011/builders/clang-cmake-mips/builds/10258/steps/ninja check 1/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 <mailto: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 <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
>>     <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
>>     <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
>>     <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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>>
>


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