X86: Tail calls with function pointer indirection not folded when more than 1 parameter is passed

[llvmbot]

Bugzilla Link	50042
Version	trunk
OS	All
Attachments	reproducer
Reporter	LLVM Bugzilla Contributor
CC	@topperc,@haberman,@RKSimon,@phoebewang,@rotateright

Extended Description

The following snippet produces the assembly with function pointer table indexing folded into JMP:

typedef void (FnTy3) (void );

void dispatch15(FnTy3 *tab, intptr_t i) { tabi; }

_dispatch15: ## @​dispatch15

%bb.0: ## %entry

jmpq    *(%rdi,%rsi,8)                  ## TAILCALL

If the called function takes at least 2 parameters, indexing isn't folded.

typedef void (FnTy2) (void , intptr_t);

void dispatch1(FnTy2 *tab, intptr_t i) { tab[i](tab, i); }

_dispatch1: ## @​dispatch1

%bb.0: ## %entry

movq    (%rdi,%rsi,8), %rax
jmpq    *%rax                           ## TAILCALL

[llvmbot]

https://reviews.llvm.org/D101718

[llvmbot]

dispatch15 DAG after X86DAGToDAGISel::PreprocessISelDAG

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dispatch1 DAG after X86DAGToDAGISel::PreprocessISelDAG craig.topper@gmail.com

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dipatch1 and dispatch15 have a different DAG shape after X86DAGToDAGISel::PreprocessISelDAG (see attachments). There's an optimisation to "try moving call address load from outside callseq_start to just before the call to allow it to be folded." It doesn't handle a multi-parameter case for tail calls.

The following patch fixes it for me:

@@ -778,11 +778,20 @@ static bool isCalleeLoad(SDValue Callee, SDValue &Chain, bool HasCallSeq) { LD->getExtensionType() != ISD::NON_EXTLOAD) return false;

• // Now let's find the callseq_start.
• while (HasCallSeq && Chain.getOpcode() != ISD::CALLSEQ_START) {
• if (!Chain.hasOneUse())
• return false;
• Chain = Chain.getOperand(0);
• if (HasCallSeq) {
• // Now let's find the callseq_start.
• while (Chain.getOpcode() != ISD::CALLSEQ_START) {
• if (!Chain.hasOneUse())
• return false;
• Chain = Chain.getOperand(0);
• }
• } else if (Chain.getOpcode() == ISD::CopyToReg) {
• // Locate first CopyToReg in the sequence of CopyToReg-s.
• while (Chain.getOperand(0).getOpcode() == ISD::CopyToReg) {
• if (!Chain.hasOneUse())
• return false;
• Chain = Chain.getOperand(0);
• } }

[llvmbot]

assigned to @topperc