[llvm] [X86] Improve illegal return type handling in FastISel (PR #186723)
via llvm-commits
llvm-commits at lists.llvm.org
Sun Mar 15 18:52:41 PDT 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-x86
Author: Luo Yuanke (LuoYuanke)
<details>
<summary>Changes</summary>
Previously, FastISel would fall back to DAG ISel for any illegal return
type. This change adds a more precise check to determine if the ABI
requires a type conversion that FastISel cannot handle.
For example, bfloat is returned as f16 in XMM0, but FastISel would
assign f32 register type and store it in FuncInfo.ValueMap, causing DAG
to incorrectly perform type conversion from f32 to bfloat later.
However, i1 is promoted to i8 and returned as i8 per the ABI, so FastISel
can safely lower it without switching to DAGISel. This change enables
FastISel to handle such cases properly.
---
Full diff: https://github.com/llvm/llvm-project/pull/186723.diff
2 Files Affected:
- (modified) llvm/lib/Target/X86/X86FastISel.cpp (+18-4)
- (added) llvm/test/CodeGen/X86/i1-fast-isel.ll (+106)
``````````diff
diff --git a/llvm/lib/Target/X86/X86FastISel.cpp b/llvm/lib/Target/X86/X86FastISel.cpp
index 0093a6b29d226..67e5a2ec869c4 100644
--- a/llvm/lib/Target/X86/X86FastISel.cpp
+++ b/llvm/lib/Target/X86/X86FastISel.cpp
@@ -3200,11 +3200,25 @@ bool X86FastISel::fastLowerCall(CallLoweringInfo &CLI) {
bool Is64Bit = Subtarget->is64Bit();
bool IsWin64 = Subtarget->isCallingConvWin64(CC);
- // If the return type is illegal, don't bother to promote it, just fall back
- // to DAG ISel.
+ // If the return type is illegal, check if the ABI requires a type conversion
+ // that FastISel cannot handle. Fall back to DAG ISel in such cases.
+ // For example, bfloat is returned as f16 in XMM0, however FastISel would
+ // assign f32 register type and store it in FuncInfo.ValueMap. This would
+ // cause DAG incorrectly perform type conversion from f32 to bfloat after get
+ // the value from FuncInfo.ValueMap.
+ // However, i1 is promoted to i8 and return i8 defined by ABI, so FastISel can
+ // lower it without switching to DAGISel.
MVT RetVT;
- if (!isTypeLegal(CLI.RetTy, RetVT) && !CLI.RetTy->isVoidTy())
- return false;
+ if (!isTypeLegal(CLI.RetTy, RetVT) && !CLI.RetTy->isVoidTy()) {
+ EVT RetEVT = EVT::getEVT(CLI.RetTy, /*HandleUnknown=*/true);
+ if (RetEVT == MVT::Other)
+ return false; // Unknown type, let DAG ISel handle it.
+ EVT RetABIEVT = TLI.getRegisterTypeForCallingConv(CLI.RetTy->getContext(),
+ CLI.CallConv, RetEVT);
+ MVT RegVT = TLI.getRegisterType(CLI.RetTy->getContext(), RetEVT);
+ if (RetABIEVT != RegVT)
+ return false;
+ }
// Call / invoke instructions with NoCfCheck attribute require special
// handling.
diff --git a/llvm/test/CodeGen/X86/i1-fast-isel.ll b/llvm/test/CodeGen/X86/i1-fast-isel.ll
new file mode 100644
index 0000000000000..1f111129de13c
--- /dev/null
+++ b/llvm/test/CodeGen/X86/i1-fast-isel.ll
@@ -0,0 +1,106 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc --fast-isel < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+
+define i8 @test_direct_call(ptr %f) nounwind {
+; CHECK-LABEL: test_direct_call:
+; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: pushq %rax
+; CHECK-NEXT: callq foo at PLT
+; CHECK-NEXT: movzbl %al, %edi
+; CHECK-NEXT: callq bar at PLT
+; CHECK-NEXT: popq %rcx
+; CHECK-NEXT: retq
+entry:
+ %call = call i1 @foo(ptr %f)
+ %call2 = call zeroext i8 @bar(i1 %call)
+ ret i8 %call2
+}
+
+define i8 @test_fast_direct_call(ptr %f) nounwind {
+; CHECK-LABEL: test_fast_direct_call:
+; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: pushq %rax
+; CHECK-NEXT: callq foo_fast at PLT
+; CHECK-NEXT: movzbl %al, %edi
+; CHECK-NEXT: callq bar at PLT
+; CHECK-NEXT: popq %rcx
+; CHECK-NEXT: retq
+entry:
+ %call = call fastcc i1 @foo_fast(ptr %f)
+ %call2 = call zeroext i8 @bar(i1 %call)
+ ret i8 %call2
+}
+
+define i8 @test_indirect_all(ptr %fptr, ptr %f) nounwind {
+; CHECK-LABEL: test_indirect_all:
+; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: pushq %rbx
+; CHECK-NEXT: movq %rdi, %rbx
+; CHECK-NEXT: movq %rsi, %rdi
+; CHECK-NEXT: callq foo at PLT
+; CHECK-NEXT: movzbl %al, %edi
+; CHECK-NEXT: callq *%rbx
+; CHECK-NEXT: popq %rbx
+; CHECK-NEXT: retq
+entry:
+ %call = call i1 @foo(ptr %f)
+ %call2 = call zeroext i8 %fptr(i1 %call)
+ ret i8 %call2
+}
+
+define i8 @test_indirect_all2(ptr %fptr, ptr %f, i1 %cond) nounwind {
+; CHECK-LABEL: test_indirect_all2:
+; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: pushq %rbp
+; CHECK-NEXT: pushq %rbx
+; CHECK-NEXT: pushq %rax
+; CHECK-NEXT: movl %edx, %ebp
+; CHECK-NEXT: movq %rdi, %rbx
+; CHECK-NEXT: movq %rsi, %rdi
+; CHECK-NEXT: callq foo at PLT
+; CHECK-NEXT: testb $1, %bpl
+; CHECK-NEXT: je .LBB3_2
+; CHECK-NEXT: # %bb.1: # %exit
+; CHECK-NEXT: movzbl %al, %edi
+; CHECK-NEXT: callq *%rbx
+; CHECK-NEXT: jmp .LBB3_3
+; CHECK-NEXT: .LBB3_2: # %exit2
+; CHECK-NEXT: movb $3, %al
+; CHECK-NEXT: .LBB3_3: # %exit2
+; CHECK-NEXT: addq $8, %rsp
+; CHECK-NEXT: popq %rbx
+; CHECK-NEXT: popq %rbp
+; CHECK-NEXT: retq
+entry:
+ %call = call i1 @foo(ptr %f)
+ br i1 %cond, label %exit, label %exit2
+
+exit:
+ %call2 = call zeroext i8 %fptr(i1 %call)
+ ret i8 %call2
+
+exit2:
+ ret i8 3
+}
+
+
+define i8 @test_fast_indirect_all(ptr %fptr, ptr %f) nounwind {
+; CHECK-LABEL: test_fast_indirect_all:
+; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: pushq %rbx
+; CHECK-NEXT: movq %rdi, %rbx
+; CHECK-NEXT: movq %rsi, %rdi
+; CHECK-NEXT: callq foo at PLT
+; CHECK-NEXT: movzbl %al, %edi
+; CHECK-NEXT: callq *%rbx
+; CHECK-NEXT: popq %rbx
+; CHECK-NEXT: retq
+entry:
+ %call = call fastcc i1 @foo(ptr %f)
+ %call2 = call zeroext i8 %fptr(i1 %call)
+ ret i8 %call2
+}
+
+declare i1 @foo(ptr %f)
+declare zeroext i8 @bar(i1)
+declare fastcc i1 @foo_fast(ptr %f)
``````````
</details>
https://github.com/llvm/llvm-project/pull/186723
More information about the llvm-commits
mailing list