[llvm] [Xtensa] Support 'f' Inline Assembly Constraint (PR #202345)

[Alexey Gerenkov via llvm-commits]

https://github.com/gerekon created https://github.com/llvm/llvm-project/pull/202345

This adds the 'f' inline assembly constraint, as supported by GCC. An 'f'-constrained operand is passed in a floating point register.

>From 455f940eda3fec6a0e71143dd37f06cfe5ef4556 Mon Sep 17 00:00:00 2001
From: Alexey Gerenkov <alexey at espressif.com>
Date: Mon, 8 Jun 2026 16:29:38 +0300
Subject: [PATCH] [Xtensa] Support 'f' Inline Assembly Constraint

This adds the 'f' inline assembly constraint, as supported by GCC. An
'f'-constrained operand is passed in a floating point register.
---
 llvm/lib/Target/Xtensa/XtensaISelLowering.cpp |  8 +++++++
 .../CodeGen/Xtensa/inline-asm-constraints.ll  | 23 +++++++++++++++++++
 .../test/CodeGen/Xtensa/inline-asm-invalid.ll | 11 ++-------
 3 files changed, 33 insertions(+), 9 deletions(-)
 create mode 100644 llvm/test/CodeGen/Xtensa/inline-asm-constraints.ll

diff --git a/llvm/lib/Target/Xtensa/XtensaISelLowering.cpp b/llvm/lib/Target/Xtensa/XtensaISelLowering.cpp
index 923afb2ec1b3d..fbe497cc66056 100644
--- a/llvm/lib/Target/Xtensa/XtensaISelLowering.cpp
+++ b/llvm/lib/Target/Xtensa/XtensaISelLowering.cpp
@@ -287,6 +287,7 @@ XtensaTargetLowering::getConstraintType(StringRef Constraint) const {
   if (Constraint.size() == 1) {
     switch (Constraint[0]) {
     case 'r':
+    case 'f':
       return C_RegisterClass;
     default:
       break;
@@ -316,6 +317,10 @@ XtensaTargetLowering::getSingleConstraintMatchWeight(
     if (Ty->isIntegerTy())
       Weight = CW_Register;
     break;
+  case 'f':
+    if (Ty->isFloatingPointTy())
+      Weight = CW_Register;
+    break;
   }
   return Weight;
 }
@@ -330,6 +335,9 @@ XtensaTargetLowering::getRegForInlineAsmConstraint(
       break;
     case 'r': // General-purpose register
       return std::make_pair(0U, &Xtensa::ARRegClass);
+    case 'f': // Floating-point register
+      if (Subtarget.hasSingleFloat())
+        return std::make_pair(0U, &Xtensa::FPRRegClass);
     }
   }
   return TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
diff --git a/llvm/test/CodeGen/Xtensa/inline-asm-constraints.ll b/llvm/test/CodeGen/Xtensa/inline-asm-constraints.ll
new file mode 100644
index 0000000000000..7dbb0f07debda
--- /dev/null
+++ b/llvm/test/CodeGen/Xtensa/inline-asm-constraints.ll
@@ -0,0 +1,23 @@
+; RUN: llc -mtriple=xtensa  -mcpu=esp32 -verify-machineinstrs < %s \
+; RUN:   | FileCheck -check-prefix=Xtensa %s
+
+
+ at gf = external global float
+
+define float @constraint_f_float(float %a) nounwind {
+; Xtensa-LABEL: constraint_f_float:
+; Xtensa:       # %bb.0:
+; Xtensa-NEXT:    entry a1, 32
+; Xtensa-NEXT:    wfr f8, a2
+; Xtensa-NEXT:    l32r a8, .LCPI0_0
+; Xtensa-NEXT:    lsi f9, a8, 0
+; Xtensa-NEXT:    #APP
+; Xtensa-NEXT:    add.s f8, f8, f9
+; Xtensa-NEXT:    #NO_APP
+; Xtensa-NEXT:    rfr a2, f8
+; Xtensa-NEXT:    retw
+  %1 = load float, float* @gf
+  %2 = tail call float asm "add.s $0, $1, $2", "=f,f,f"(float %a, float %1)
+  ret float %2
+}
+
diff --git a/llvm/test/CodeGen/Xtensa/inline-asm-invalid.ll b/llvm/test/CodeGen/Xtensa/inline-asm-invalid.ll
index cb9cd83fbc156..2da5fcc3fb97b 100644
--- a/llvm/test/CodeGen/Xtensa/inline-asm-invalid.ll
+++ b/llvm/test/CodeGen/Xtensa/inline-asm-invalid.ll
@@ -1,14 +1,7 @@
-; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
-; RUN: not llc --mtriple=xtensa < %s 2>&1 | FileCheck %s
+; RUN: not llc -mtriple=xtensa -mcpu=generic < %s 2>&1 | FileCheck %s
 
+; CHECK: error: could not allocate input reg for constraint 'f'
 define void @constraint_f() nounwind {
-; CHECK: error: unknown asm constraint 'f'
   tail call void asm "add.s f0, f1, $0", "f"(float 0.0)
   ret void
 }
-
-define i32 @register_a100(i32 %a) nounwind {
-; CHECK: error: could not allocate input reg for constraint '{$a100}'
-  %1 = tail call i32 asm "addi $0, $1, 1", "=r,{$a100}"(i32 %a)
-  ret i32 %1
-}