[llvm-branch-commits] [llvm] [CodeGen] Report an error for a direct inline asm output in memory (PR #229628)
Bill Wendling via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Fri Oct 16 01:55:52 PDT 2026
https://github.com/isanbard updated https://github.com/llvm/llvm-project/pull/229628
>From 095f83b8a7422ee2bd8869dea9fac1ab0e3bf8ce Mon Sep 17 00:00:00 2001
From: Bill Wendling <isanbard at gmail.com>
Date: Tue, 6 Oct 2026 06:12:08 -0700
Subject: [PATCH 1/2] [CodeGen] Report an error for a direct inline asm output
in memory
An inline asm output returned by value has no memory to write to, yet a
constraint such as "=rm" picks memory, the most general constraint, as
does "=m". SelectionDAG asserted on that ("Can only indirectify direct
input operands!"), and GlobalISel dereferenced a null pointer. Clang
never emits such an output, since it passes the address of a memory
output, but other IR can. Report "cannot handle direct memory outputs
yet for constraint 'm'" instead, like the other inline asm errors there.
Assisted-by: Claude Opus 5.5
---
.../CodeGen/GlobalISel/InlineAsmLowering.cpp | 10 ++++++
.../SelectionDAG/SelectionDAGBuilder.cpp | 22 +++++++++----
.../inline-asm-direct-mem-output-error.ll | 13 ++++++++
.../X86/inline-asm-direct-mem-output-error.ll | 31 +++++++++++++++++++
4 files changed, 70 insertions(+), 6 deletions(-)
create mode 100644 llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
create mode 100644 llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
diff --git a/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp b/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
index 19eae930cc26b1..5d66cb5e332276 100644
--- a/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
@@ -337,6 +337,16 @@ bool InlineAsmLowering::lowerInlineAsm(
switch (OpInfo.Type) {
case InlineAsm::isOutput:
if (OpInfo.ConstraintType == TargetLowering::C_Memory) {
+ // A direct output is the call's result, with no address to write to.
+ if (!OpInfo.isIndirect) {
+ emitInlineAsmError(MIRBuilder, Call,
+ "cannot handle direct memory outputs yet for "
+ "constraint '" +
+ Twine(OpInfo.ConstraintCode) + "'",
+ GetOrCreateVRegs(Call));
+ return true;
+ }
+
const InlineAsm::ConstraintCode ConstraintID =
TLI->getInlineAsmMemConstraint(OpInfo.ConstraintCode);
assert(ConstraintID != InlineAsm::ConstraintCode::Unknown &&
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index b595de3734bb9f..e994b6af23eb04 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -10504,8 +10504,9 @@ constructOperandInfo(ConstraintDecisionInfo &Info,
return false;
}
-/// Compute which constraint option to use for each operand.
-static void
+/// Compute which constraint option to use for each operand. Returns true (and
+/// sets Info.ErrorMsg) on failure.
+static bool
computeConstraintToUse(ConstraintDecisionInfo &Info, const CallBase &Call,
TargetLowering::AsmOperandInfoVector &TargetConstraints,
SelectionDAGBuilder &Builder, const TargetLowering &TLI,
@@ -10567,9 +10568,14 @@ computeConstraintToUse(ConstraintDecisionInfo &Info, const CallBase &Call,
// need to provide an address for the memory input.
if (OpInfo.ConstraintType == TargetLowering::C_Memory &&
!OpInfo.isIndirect) {
- assert((OpInfo.isMultipleAlternative ||
- (OpInfo.Type == InlineAsm::isInput)) &&
- "Can only indirectify direct input operands!");
+ // Only an input has a value to store to memory. A direct output, which
+ // Clang never emits with a memory constraint but other IR can, would
+ // need a stack slot to be reloaded after the asm.
+ if (!OpInfo.isMultipleAlternative && OpInfo.Type != InlineAsm::isInput) {
+ Info.ErrorMsg << "cannot handle direct memory outputs yet for "
+ << "constraint '" << OpInfo.ConstraintCode << "'";
+ return true;
+ }
// Memory operands really want the address of the value.
Info.Chain = getAddressForMemoryInput(Info.Chain, Builder.getCurSDLoc(),
@@ -10582,6 +10588,8 @@ computeConstraintToUse(ConstraintDecisionInfo &Info, const CallBase &Call,
OpInfo.isIndirect = true;
}
}
+
+ return false;
}
/// Prepare DAG-level operands. As part of this, assign virtual and physical
@@ -10875,7 +10883,9 @@ determineConstraints(ConstraintDecisionInfo &Info,
Info.Chain = Builder.lowerStartEH(Info.Chain, EHPadBB, Info.BeginLabel);
// Second pass: Compute which constraint option to use.
- computeConstraintToUse(Info, Call, TargetConstraints, Builder, TLI, TM, DAG);
+ if (computeConstraintToUse(Info, Call, TargetConstraints, Builder, TLI, TM,
+ DAG))
+ return true;
// AsmNodeOperands - The operands for the ISD::INLINEASM node.
Info.AsmNodeOperands.push_back(SDValue()); // reserve space for input chain
diff --git a/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll b/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
new file mode 100644
index 00000000000000..7ebe9f006943f9
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
@@ -0,0 +1,13 @@
+; RUN: not llc -mtriple=aarch64 -filetype=null < %s 2>&1 | FileCheck %s
+; RUN: not llc -mtriple=aarch64 -global-isel -global-isel-abort=1 \
+; RUN: -filetype=null < %s 2>&1 | FileCheck %s
+
+; A direct output is the asm's result, so there is no memory to write it to.
+; Picking a memory constraint for one is an error, not a crash, with either
+; instruction selector.
+
+; CHECK: error: cannot handle direct memory outputs yet for constraint 'm'
+define i64 @rm_output() {
+ %r = call i64 asm "# $0", "=rm"()
+ ret i64 %r
+}
diff --git a/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll b/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
new file mode 100644
index 00000000000000..3f3a9168de38f8
--- /dev/null
+++ b/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
@@ -0,0 +1,31 @@
+; RUN: not llc -mtriple=x86_64-unknown-linux-gnu -O0 -filetype=null < %s 2>&1 \
+; RUN: | FileCheck --check-prefix=O0 --implicit-check-not=error: %s
+; RUN: not llc -mtriple=x86_64-unknown-linux-gnu -filetype=null < %s 2>&1 \
+; RUN: | FileCheck --check-prefix=O2 --implicit-check-not=error: %s
+
+; A direct output is the asm's result, so there is no memory to write it to.
+; Picking a memory constraint for one is an error, not a crash.
+
+; "rm" picks memory, the most general constraint.
+; O0: error: cannot handle direct memory outputs yet for constraint 'm'
+; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+define i32 @rm_i32() {
+ %r = call i32 asm "# $0", "=rm"()
+ ret i32 %r
+}
+
+; The same for a value no 'r' register can hold.
+; O0: error: cannot handle direct memory outputs yet for constraint 'm'
+; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+define x86_fp80 @rm_x86_fp80() {
+ %r = call x86_fp80 asm "# $0", "=rm"()
+ ret x86_fp80 %r
+}
+
+; "=m" has no other choice.
+; O0: error: cannot handle direct memory outputs yet for constraint 'm'
+; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+define i32 @m_i32() {
+ %r = call i32 asm "# $0", "=m"()
+ ret i32 %r
+}
>From 8a0c12bc816aca775c88d485dec185c1c3106a06 Mon Sep 17 00:00:00 2001
From: Bill Wendling <isanbard at gmail.com>
Date: Thu, 15 Oct 2026 18:20:52 -0700
Subject: [PATCH 2/2] [CodeGen] Address review: error for "=m|r" direct outputs
too
A direct output whose chosen alternative is memory, e.g. "=m|r", still
crashed SelectionDAG, which exempted multiple-alternative operands from the
check and then asked for the address of an output with no value. Drop the
exemption and test "=m|r" with both instruction selectors. Name the
selector explicitly in the RUN lines rather than relying on -O levels,
since X86 GlobalISel can't lower inline asm and both X86 lines ran
SelectionDAG anyway. Drop "yet" from the error: Clang never emits a
direct memory output, so there are no plans to support one.
Assisted-by: Claude Opus 5.5
---
.../CodeGen/GlobalISel/InlineAsmLowering.cpp | 2 +-
.../SelectionDAG/SelectionDAGBuilder.cpp | 9 +++----
.../inline-asm-direct-mem-output-error.ll | 14 ++++++++---
.../X86/inline-asm-direct-mem-output-error.ll | 24 +++++++++++--------
4 files changed, 31 insertions(+), 18 deletions(-)
diff --git a/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp b/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
index 5d66cb5e332276..210f693f7bf583 100644
--- a/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/InlineAsmLowering.cpp
@@ -340,7 +340,7 @@ bool InlineAsmLowering::lowerInlineAsm(
// A direct output is the call's result, with no address to write to.
if (!OpInfo.isIndirect) {
emitInlineAsmError(MIRBuilder, Call,
- "cannot handle direct memory outputs yet for "
+ "cannot handle direct memory outputs for "
"constraint '" +
Twine(OpInfo.ConstraintCode) + "'",
GetOrCreateVRegs(Call));
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index e994b6af23eb04..c15e96738752cc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -10570,10 +10570,11 @@ computeConstraintToUse(ConstraintDecisionInfo &Info, const CallBase &Call,
!OpInfo.isIndirect) {
// Only an input has a value to store to memory. A direct output, which
// Clang never emits with a memory constraint but other IR can, would
- // need a stack slot to be reloaded after the asm.
- if (!OpInfo.isMultipleAlternative && OpInfo.Type != InlineAsm::isInput) {
- Info.ErrorMsg << "cannot handle direct memory outputs yet for "
- << "constraint '" << OpInfo.ConstraintCode << "'";
+ // need a stack slot to be reloaded after the asm. That includes an
+ // output whose chosen alternative is memory, e.g. "=m|r".
+ if (OpInfo.Type != InlineAsm::isInput) {
+ Info.ErrorMsg << "cannot handle direct memory outputs for constraint '"
+ << OpInfo.ConstraintCode << "'";
return true;
}
diff --git a/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll b/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
index 7ebe9f006943f9..12aa4a46da9989 100644
--- a/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
+++ b/llvm/test/CodeGen/AArch64/inline-asm-direct-mem-output-error.ll
@@ -1,13 +1,21 @@
-; RUN: not llc -mtriple=aarch64 -filetype=null < %s 2>&1 | FileCheck %s
+; RUN: not llc -mtriple=aarch64 -global-isel=0 -filetype=null < %s 2>&1 \
+; RUN: | FileCheck --implicit-check-not=error: %s
; RUN: not llc -mtriple=aarch64 -global-isel -global-isel-abort=1 \
-; RUN: -filetype=null < %s 2>&1 | FileCheck %s
+; RUN: -filetype=null < %s 2>&1 | FileCheck --implicit-check-not=error: %s
; A direct output is the asm's result, so there is no memory to write it to.
; Picking a memory constraint for one is an error, not a crash, with either
; instruction selector.
-; CHECK: error: cannot handle direct memory outputs yet for constraint 'm'
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
define i64 @rm_output() {
%r = call i64 asm "# $0", "=rm"()
ret i64 %r
}
+
+; With multiple alternatives, the memory one is chosen here, too.
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
+define i64 @multi_alt() {
+ %r = call i64 asm "# $0", "=m|r"()
+ ret i64 %r
+}
diff --git a/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll b/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
index 3f3a9168de38f8..00135acfdc4769 100644
--- a/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
+++ b/llvm/test/CodeGen/X86/inline-asm-direct-mem-output-error.ll
@@ -1,31 +1,35 @@
-; RUN: not llc -mtriple=x86_64-unknown-linux-gnu -O0 -filetype=null < %s 2>&1 \
-; RUN: | FileCheck --check-prefix=O0 --implicit-check-not=error: %s
-; RUN: not llc -mtriple=x86_64-unknown-linux-gnu -filetype=null < %s 2>&1 \
-; RUN: | FileCheck --check-prefix=O2 --implicit-check-not=error: %s
+; X86 GlobalISel can't lower inline asm, so only SelectionDAG is tested here;
+; see the AArch64 test for both instruction selectors.
+; RUN: not llc -mtriple=x86_64-unknown-linux-gnu -global-isel=0 \
+; RUN: -filetype=null < %s 2>&1 | FileCheck --implicit-check-not=error: %s
; A direct output is the asm's result, so there is no memory to write it to.
; Picking a memory constraint for one is an error, not a crash.
; "rm" picks memory, the most general constraint.
-; O0: error: cannot handle direct memory outputs yet for constraint 'm'
-; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
define i32 @rm_i32() {
%r = call i32 asm "# $0", "=rm"()
ret i32 %r
}
; The same for a value no 'r' register can hold.
-; O0: error: cannot handle direct memory outputs yet for constraint 'm'
-; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
define x86_fp80 @rm_x86_fp80() {
%r = call x86_fp80 asm "# $0", "=rm"()
ret x86_fp80 %r
}
; "=m" has no other choice.
-; O0: error: cannot handle direct memory outputs yet for constraint 'm'
-; O2: error: cannot handle direct memory outputs yet for constraint 'm'
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
define i32 @m_i32() {
%r = call i32 asm "# $0", "=m"()
ret i32 %r
}
+
+; With multiple alternatives, the memory one is chosen here, too.
+; CHECK: error: cannot handle direct memory outputs for constraint 'm'
+define i32 @multi_alt() {
+ %r = call i32 asm "# $0", "=m|r"()
+ ret i32 %r
+}
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