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BlockGenerator: Be less agressive with deleting dead instructions 

We now only delete trivially dead instructions in the BB we copy (copyBB), but
not in any other BB.  Only for copyBB we know that there will _never_ be any
future uses of instructions that have no use after copyBB has been generated.
Other instructions in the AST that have been generated by IslNodeBuilder may
look dead at the moment, but may possibly still be referenced by GlobalMaps. If
we delete them now, later uses would break surprisingly.

We do not have a test case that breaks due to us deleting too many instructions.
This issue was found by inspection.

llvm-svn: 248688
This commit is contained in:
Tobias Grosser 2015-09-27 19:50:16 +00:00
parent a43b6e935c
commit 0722a1e5d5
3 changed files with 46 additions and 8 deletions
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38
polly/lib/CodeGen/BlockGenerators.cpp
View File

@ -285,6 +285,28 @@ void BlockGenerator::copyInstruction(ScopStmt &Stmt, const Instruction *Inst,
copyInstScalar(Stmt, Inst, BBMap, LTS);
}
/// @brief Remove trivially dead instructions from BB
///
/// This function drops trivially dead instructions from a basic block. It
/// on purpose does _not_ recurse into other BBs even if the deletion of
/// instructions in this basic block can make instructions in other basic blocks
/// triviall dead.
static void simplifyInstsInBlockOnly(BasicBlock *BB) {
auto BI = --BB->end(), BE = BB->begin();
bool Exit = false;
while (!Exit) {
auto ToRemove = BI;
if (BI != BE)
BI--;
else
Exit = true;
if (!isInstructionTriviallyDead(ToRemove))
continue;
ToRemove->eraseFromParent();
}
}
void BlockGenerator::copyStmt(ScopStmt &Stmt, LoopToScevMapT &LTS,
isl_id_to_ast_expr *NewAccesses) {
assert(Stmt.isBlockStmt() &&
@ -296,7 +318,13 @@ void BlockGenerator::copyStmt(ScopStmt &Stmt, LoopToScevMapT &LTS,
copyBB(Stmt, BB, BBMap, LTS, NewAccesses);
auto CopyBB = Builder.GetInsertBlock();
SimplifyInstructionsInBlock(CopyBB, nullptr);
// Delete trivially dead instructions in CopyBB, but not in any other BB.
// Only for copyBB we know that there will _never_ be any future uses of
// instructions that have no use after copyBB has finished. Other instructions
// in the AST that have been generated by IslNodeBuilder may look dead at
// the moment, but may possibly still be referenced by GlobalMaps. If we
// delete them now, later uses would break surprisingly.
simplifyInstsInBlockOnly(CopyBB);
Builder.SetInsertPoint(CopyBB->getTerminator());
}
@ -1090,8 +1118,14 @@ void RegionGenerator::copyStmt(ScopStmt &Stmt, LoopToScevMapT &LTS,
LTS[L] = SE.getUnknown(LoopPHI);
}
// Delete trivially dead instructions in CopyBB, but not in any other BB.
// Only for copyBB we know that there will _never_ be any future uses of
// instructions that have no use after copyBB has finished. Other instructions
// in the AST that have been generated by IslNodeBuilder may look dead at
// the moment, but may possibly still be referenced by GlobalMaps. If we
// delete them now, later uses would break surprisingly.
for (auto *BB : SeenBlocks)
SimplifyInstructionsInBlock(BlockMap[BB], nullptr);
simplifyInstsInBlockOnly(BlockMap[BB]);
// Reset the old insert point for the build.
Builder.SetInsertPoint(ExitBBCopy->begin());

3
polly/test/Isl/CodeGen/MemAccess/codegen_address_space.ll
View File

@ -40,4 +40,5 @@ for.end: ; preds = %for.cond
}
; CHECK: %polly.access.cast.A = bitcast [100 x i32] addrspace(5)* %A to i32 addrspace(5)*
; CHECK: %tmp2_p_scalar_ = load i32, i32 addrspace(5)* %polly.access.cast.A, align 4, !alias.scope !0, !noalias !2
; CHECK: %polly.access.A = getelementptr i32, i32 addrspace(5)* %polly.access.cast.A, i64 0
; CHECK: %tmp2_p_scalar_ = load i32, i32 addrspace(5)* %polly.access.A, align 4, !alias.scope !0, !noalias !2

13
polly/test/Isl/CodeGen/non-affine-phi-node-expansion-3.ll
View File

@ -13,9 +13,12 @@ loop:
br i1 %cond0, label %branch1, label %backedge
; CHECK-LABEL: polly.stmt.loop:
; CHECK-NEXT: store float 3.000000e+00, float* %merge.phiops
; CHECK-NEXT: store float 3.000000e+00, float* %val1.s2a
; CHECK-NEXT: store float 3.000000e+00, float* %val2.s2a
; CHECK-NEXT: %p_val0 = fadd float 1.000000e+00, 2.000000e+00
; CHECK-NEXT: %p_val1 = fadd float 1.000000e+00, 2.000000e+00
; CHECK-NEXT: %p_val2 = fadd float 1.000000e+00, 2.000000e+00
; CHECK-NEXT: store float %p_val0, float* %merge.phiops
; CHECK-NEXT: store float %p_val1, float* %val1.s2a
; CHECK-NEXT: store float %p_val2, float* %val2.s2a
; FIXME -> The last two writes are not really needed and can be dropped if the
; incoming block of the PHI and the value that is used share the same
@ -25,13 +28,13 @@ branch1:
br i1 %cond1, label %branch2, label %backedge
; CHECK-LABEL: polly.stmt.branch1:
; CHECK-NEXT: store float 3.000000e+00, float* %merge.phiops
; CHECK-NEXT: store float %p_val1, float* %merge.phiops
branch2:
br label %backedge
; CHECK-LABEL: polly.stmt.branch2:
; CHECK-NEXT: store float 3.000000e+00, float* %merge.phiops
; CHECK-NEXT: store float %p_val2, float* %merge.phiops
backedge:
%merge = phi float [%val0, %loop], [%val1, %branch1], [%val2, %branch2]