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[FastISel] Don't select a CallInst as a BasicBlock in the SelectionDAG fallback if it has bundled ops #162895

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[FastISel] Don't select a CallInst as a BasicBlock in the SelectionDAG fallback if it has bundled ops #162895

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8 changes: 8 additions & 0 deletions llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1825,6 +1825,14 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {

reportFastISelFailure(*MF, *ORE, R, EnableFastISelAbort > 2);

// If the call has operand bundles, then it's best if they are handled
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I don't understand the flow here. These seems pretty far removed from where the problem is?

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The root cause of the problem is that SelectionDAG is designed to lower CallInst and its bundled ops together: for example, both the indirect call and the address-of GlobalValue that are generated in CF Guard (which can then be lowered as a call to the address of a GlobalValue, instead of reading the value and then calling to a register).

I'm not sure why SelectionDAGISel has this special case here (I assume to try to lower calls via SelectionDAG but then allow the rest of the block to be handled by FastISel), but it breaks that assumption and causes suboptimal codegen.

// together with the call instead of selecting the call as its own
// block.
if (cast<CallInst>(Inst)->hasOperandBundles()) {
NumFastIselFailures += NumFastIselRemaining;
break;
}

if (!Inst->getType()->isVoidTy() && !Inst->getType()->isTokenTy() &&
!Inst->use_empty()) {
Register &R = FuncInfo->ValueMap[Inst];
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9 changes: 6 additions & 3 deletions llvm/test/CodeGen/X86/cfguard-checks.ll
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@@ -1,7 +1,9 @@
; RUN: llc < %s -mtriple=i686-pc-windows-msvc | FileCheck %s -check-prefix=X86
; RUN: llc < %s -mtriple=x86_64-pc-windows-msvc | FileCheck %s -check-prefixes=X64,X64_MSVC
; RUN: llc < %s -mtriple=x86_64-pc-windows-msvc | FileCheck %s -check-prefixes=X64,X64_MSVC,X64_SELDAG
; RUN: llc < %s --fast-isel -mtriple=x86_64-pc-windows-msvc | FileCheck %s -check-prefixes=X64,X64_MSVC,X64_FISEL
; RUN: llc < %s -mtriple=i686-w64-windows-gnu | FileCheck %s -check-prefixes=X86,X86_MINGW
; RUN: llc < %s -mtriple=x86_64-w64-windows-gnu | FileCheck %s -check-prefixes=X64,X64_MINGW
; RUN: llc < %s -mtriple=x86_64-w64-windows-gnu | FileCheck %s -check-prefixes=X64,X64_MINGW,X64_SELDAG
; RUN: llc < %s --fast-isel -mtriple=x86_64-w64-windows-gnu | FileCheck %s -check-prefixes=X64,X64_MINGW,X64_FISEL
; Control Flow Guard is currently only available on Windows

; Test that Control Flow Guard checks are correctly added when required.
Expand All @@ -27,7 +29,8 @@ entry:
; X64-LABEL: func_guard_nocf
; X64: leaq target_func(%rip), %rax
; X64-NOT: __guard_dispatch_icall_fptr
; X64: callq *%rax
; X64_SELDAG: callq *%rax
; X64_FISEL: callq *32(%rsp)
}
attributes #0 = { "guard_nocf" }

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