MIPS: Fix asm constraints "f" and "r" for softfloat (#79116)

This include 2 fixes:
        1. Disallow 'f' for softfloat.
        2. Allow 'r' for softfloat.

Currently, 'f' is accpeted by clang, then LLVM meets an internal error.

'r' is rejected by LLVM by: couldn't allocate input reg for constraint
'r'.

Fixes: #64241, #63632

---------

Co-authored-by: Fangrui Song <i@maskray.me>
(cherry picked from commit c88beb4)

clang/lib/Basic/Targets/Mips.h

@@ -237,12 +237,14 @@ class LLVM_LIBRARY_VISIBILITY MipsTargetInfo : public TargetInfo {
     case 'r': // CPU registers.
     case 'd': // Equivalent to "r" unless generating MIPS16 code.
     case 'y': // Equivalent to "r", backward compatibility only.
-    case 'f': // floating-point registers.
     case 'c': // $25 for indirect jumps
     case 'l': // lo register
     case 'x': // hilo register pair
       Info.setAllowsRegister();
       return true;
+    case 'f': // floating-point registers.
+      Info.setAllowsRegister();
+      return FloatABI != SoftFloat;
     case 'I': // Signed 16-bit constant
     case 'J': // Integer 0
     case 'K': // Unsigned 16-bit constant

clang/test/CodeGen/Mips/inline-asm-constraints.c

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -emit-llvm -triple mips -target-feature +soft-float %s -o - | FileCheck %s --check-prefix=SOFT_FLOAT
+
+// SOFT_FLOAT: call void asm sideeffect "", "r,~{$1}"(float %1)
+void read_float(float *p) {
+  __asm__("" ::"r"(*p));
+}
+
+// SOFT_FLOAT: call void asm sideeffect "", "r,~{$1}"(double %1)
+void read_double(double *p) {
+  __asm__("" :: "r"(*p));
+}

clang/test/Sema/inline-asm-validate-mips.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple mips64 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple mips64 -target-feature +soft-float -fsyntax-only -verify=softfloat %s
+
+// expected-no-diagnostics
+
+void test_f(float p) {
+  float result = p;
+  __asm__("" :: "f"(result)); // softfloat-error{{invalid input constraint 'f' in asm}}
+}

llvm/lib/Target/Mips/MipsISelLowering.cpp

@@ -4128,14 +4128,18 @@ MipsTargetLowering::getRegForInlineAsmConstraint(const TargetRegisterInfo *TRI,
     case 'd': // Address register. Same as 'r' unless generating MIPS16 code.
     case 'y': // Same as 'r'. Exists for compatibility.
     case 'r':
-      if (VT == MVT::i32 || VT == MVT::i16 || VT == MVT::i8 || VT == MVT::i1) {
+      if ((VT == MVT::i32 || VT == MVT::i16 || VT == MVT::i8 ||
+           VT == MVT::i1) ||
+          (VT == MVT::f32 && Subtarget.useSoftFloat())) {
         if (Subtarget.inMips16Mode())
           return std::make_pair(0U, &Mips::CPU16RegsRegClass);
         return std::make_pair(0U, &Mips::GPR32RegClass);
       }
-      if (VT == MVT::i64 && !Subtarget.isGP64bit())
+      if ((VT == MVT::i64 || (VT == MVT::f64 && Subtarget.useSoftFloat())) &&
+          !Subtarget.isGP64bit())
         return std::make_pair(0U, &Mips::GPR32RegClass);
-      if (VT == MVT::i64 && Subtarget.isGP64bit())
+      if ((VT == MVT::i64 || (VT == MVT::f64 && Subtarget.useSoftFloat())) &&
+          Subtarget.isGP64bit())
         return std::make_pair(0U, &Mips::GPR64RegClass);
       // This will generate an error message
       return std::make_pair(0U, nullptr);

llvm/test/CodeGen/Mips/inlineasm-constraints-softfloat.ll

@@ -0,0 +1,48 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc -march=mips < %s | FileCheck %s --check-prefix=MIPS32
+; RUN: llc -march=mips64 < %s | FileCheck %s --check-prefix=MIPS64
+
+define dso_local void @read_double(ptr nocapture noundef readonly %0) local_unnamed_addr #0 {
+; MIPS32-LABEL: read_double:
+; MIPS32:       # %bb.0:
+; MIPS32-NEXT:    lw $2, 4($4)
+; MIPS32-NEXT:    lw $3, 0($4)
+; MIPS32-NEXT:    #APP
+; MIPS32-NEXT:    #NO_APP
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+;
+; MIPS64-LABEL: read_double:
+; MIPS64:       # %bb.0:
+; MIPS64-NEXT:    ld $2, 0($4)
+; MIPS64-NEXT:    #APP
+; MIPS64-NEXT:    #NO_APP
+; MIPS64-NEXT:    jr $ra
+; MIPS64-NEXT:    nop
+  %2 = load double, ptr %0, align 8
+  tail call void asm sideeffect "", "r,~{$1}"(double %2)
+  ret void
+}
+
+define dso_local void @read_float(ptr nocapture noundef readonly %0) local_unnamed_addr #0 {
+; MIPS32-LABEL: read_float:
+; MIPS32:       # %bb.0:
+; MIPS32-NEXT:    lw $2, 0($4)
+; MIPS32-NEXT:    #APP
+; MIPS32-NEXT:    #NO_APP
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+;
+; MIPS64-LABEL: read_float:
+; MIPS64:       # %bb.0:
+; MIPS64-NEXT:    lw $2, 0($4)
+; MIPS64-NEXT:    #APP
+; MIPS64-NEXT:    #NO_APP
+; MIPS64-NEXT:    jr $ra
+; MIPS64-NEXT:    nop
+  %2 = load float, ptr %0, align 8
+  tail call void asm sideeffect "", "r,~{$1}"(float %2)
+  ret void
+}
+
+attributes #0 = { "target-features"="+soft-float" "use-soft-float"="true" }