[X86] Allow "sibcall" optimization on call sites marked musttail
#150157
Conversation
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@llvm/pr-subscribers-backend-x86 Author: Brandt Bucher (brandtbucher) ChangesFixes #147813. Use the same code path to detect sibcall-eligibility for both Full diff: https://github.com/llvm/llvm-project/pull/150157.diff 2 Files Affected:
diff --git a/llvm/lib/Target/X86/X86ISelLoweringCall.cpp b/llvm/lib/Target/X86/X86ISelLoweringCall.cpp
index b4639ac2577e8..9835586407042 100644
--- a/llvm/lib/Target/X86/X86ISelLoweringCall.cpp
+++ b/llvm/lib/Target/X86/X86ISelLoweringCall.cpp
@@ -2095,7 +2095,12 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
isTailCall = false;
}
- if (isTailCall && !IsMustTail) {
+ if (IsMustTail && !isTailCall) {
+ report_fatal_error("failed to perform tail call elimination on a call site "
+ "marked musttail");
+ }
+
+ if (isTailCall) {
// Check if it's really possible to do a tail call.
isTailCall = IsEligibleForTailCallOptimization(CLI, CCInfo, ArgLocs,
IsCalleePopSRet);
@@ -2109,9 +2114,9 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
++NumTailCalls;
}
- if (IsMustTail && !isTailCall)
- report_fatal_error("failed to perform tail call elimination on a call "
- "site marked musttail");
+ if (IsMustTail) {
+ isTailCall = true;
+ }
assert(!(isVarArg && canGuaranteeTCO(CallConv)) &&
"Var args not supported with calling convention fastcc, ghc or hipe");
diff --git a/llvm/test/CodeGen/X86/musttail-sibling.ll b/llvm/test/CodeGen/X86/musttail-sibling.ll
new file mode 100644
index 0000000000000..91a0ae059b01a
--- /dev/null
+++ b/llvm/test/CodeGen/X86/musttail-sibling.ll
@@ -0,0 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=x86_64 < %s | FileCheck %s --check-prefix=X64
+; RUN: llc -mtriple=i686 < %s | FileCheck %s --check-prefix=X86
+
+declare void @callee1(ptr)
+define void @caller1(ptr %s) {
+; X64-LABEL: caller1:
+; X64: # %bb.0:
+; X64-NEXT: jmp callee1@PLT # TAILCALL
+;
+; X86-LABEL: caller1:
+; X86: # %bb.0:
+; X86-NEXT: jmp callee1@PLT # TAILCALL
+ musttail call void @callee1(ptr %s)
+ ret void
+}
+
+declare void @callee7(ptr)
+define void @caller7(ptr %r0, ptr %r1, ptr %r2, ptr %r3, ptr %r4, ptr %r5, ptr %s) {
+; X64-LABEL: caller7:
+; X64: # %bb.0:
+; X64-NEXT: jmp callee7@PLT # TAILCALL
+;
+; X86-LABEL: caller7:
+; X86: # %bb.0:
+; X86-NEXT: jmp callee7@PLT # TAILCALL
+ musttail call void @callee7(ptr %r0, ptr %r1, ptr %r2, ptr %r3, ptr %r4, ptr %r5, ptr %s)
+ ret void
+}
+
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I think this code to load and store all parameters in memory exists to handle cases where the parameters are changed or swapped, consider: The incoming parameters in memory and the outgoing arguments compete for the same slots in memory, and they have to be loaded and stored in some cases, and it's hard to figure out which ones require it and which do not. We even have correctness bugs when musttail combines with byval: #46402 Triggering this case requires passing marginally complicated structs in memory (>=7 params for x86_64 or ia32). |
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@rnk, maybe I'm missing something, but this PR emits the same code as the example you linked. Would you like me to add a test for it? define void @caller_flip7(ptr %r0, ptr %r1, ptr %r2, ptr %r3, ptr %r4, ptr %r5, ptr %x) {
; X64-LABEL: caller_flip7:
; X64: # %bb.0:
; X64-NEXT: movq {{[0-9]+}}(%rsp), %rax
; X64-NEXT: movq %r9, {{[0-9]+}}(%rsp)
; X64-NEXT: movq %rax, %r9
; X64-NEXT: jmp callee7@PLT # TAILCALL
musttail call void @callee7(ptr %r0, ptr %r1, ptr %r2, ptr %r3, ptr %r4, ptr %x, ptr %r5)
ret void
} |
Yep, if it already works, my concerns would be addressed. |
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Actually, I recall that Swift uses musttail in a creative way, where it doesn't require prototype mismatch, but still guarantees the tail call. There are roughly three cases:
We have a confusing tangle of booleans in this code to try to differentiate between these cases, but I think that's the true purpose of the |
Fixes #147813.
Use the same code path to detect sibcall-eligibility for both
musttailand normal tail calls.