[RISCV] Support constraint "s" (#80201)

GCC has supported a generic constraint "s" for a long time (since at
least 1992), which references a symbol or label with an optional
constant offset. "i" is a superset that also supports a constant
integer.

GCC's RISC-V port also supports a machine-specific constraint "S",
which cannot be used with a preemptible symbol. (We don't bother to
check preemptibility.) In PIC code, an external symbol is preemptible by
default, making "S" less useful if you want to create an artificial
reference for linker garbage collection, or define sections to hold
symbol addresses:

```
void fun();
// error: impossible constraint in ‘asm’ for riscv64-linux-gnu-gcc -fpie/-fpic
void foo() { asm(".reloc ., BFD_RELOC_NONE, %0" :: "S"(fun)); }
// good even if -fpie/-fpic
void foo() { asm(".reloc ., BFD_RELOC_NONE, %0" :: "s"(fun)); }
```

This patch adds support for "s". Modify https://reviews.llvm.org/D105254
("S") to handle multi-depth GEPs (https://reviews.llvm.org/D61560).

Author: MaskRay

clang/lib/Basic/Targets/RISCV.cpp

@@ -96,7 +96,8 @@ bool RISCVTargetInfo::validateAsmConstraint(
     // An address that is held in a general-purpose register.
     Info.setAllowsMemory();
     return true;
-  case 'S': // A symbolic address
+  case 's':
+  case 'S': // A symbol or label reference with a constant offset
     Info.setAllowsRegister();
     return true;
   case 'v':

clang/test/CodeGen/RISCV/riscv-inline-asm.c

@@ -45,8 +45,16 @@ void test_A(int *p) {
   asm volatile("" :: "A"(*p));
 }
 
-void test_S(void) {
-// CHECK-LABEL: define{{.*}} void @test_S()
-// CHECK: call void asm sideeffect "", "S"(ptr nonnull @f)
-  asm volatile("" :: "S"(&f));
+extern int var, arr[2][2];
+struct Pair { int a, b; } pair;
+
+// CHECK-LABEL: test_s(
+// CHECK:         call void asm sideeffect "// $0 $1 $2", "s,s,s"(ptr nonnull @var, ptr nonnull getelementptr inbounds ([2 x [2 x i32]], ptr @arr, {{.*}}), ptr nonnull @test_s)
+// CHECK:         call void asm sideeffect "// $0", "s"(ptr nonnull getelementptr inbounds (%struct.Pair, ptr @pair, {{.*}}))
+// CHECK:         call void asm sideeffect "// $0 $1 $2", "S,S,S"(ptr nonnull @var, ptr nonnull getelementptr inbounds ([2 x [2 x i32]], ptr @arr, {{.*}}), ptr nonnull @test_s)
+void test_s(void) {
+  asm("// %0 %1 %2" :: "s"(&var), "s"(&arr[1][1]), "s"(test_s));
+  asm("// %0" :: "s"(&pair.b));
+
+  asm("// %0 %1 %2" :: "S"(&var), "S"(&arr[1][1]), "S"(test_s));
 }

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

@@ -22,6 +22,14 @@ void K(int k) {
   asm volatile ("" :: "K"(AboveMax)); // expected-error{{value '32' out of range for constraint 'K'}}
 }
 
+void test_s(int i) {
+  asm("" :: "s"(test_s(0))); // expected-error{{invalid type 'void' in asm input for constraint 's'}}
+  /// Codegen error
+  asm("" :: "s"(i));
+
+  asm("" :: "S"(test_s(0))); // expected-error{{invalid type 'void' in asm input for constraint 'S'}}
+}
+
 void test_clobber_conflict(void) {
   register long x10 asm("x10");
   asm volatile("" :: "r"(x10) : "x10"); // expected-error {{conflicts with asm clobber list}}

llvm/docs/LangRef.rst

@@ -5075,7 +5075,7 @@ Some constraint codes are typically supported by all targets:
 - ``i``: An integer constant (of target-specific width). Allows either a simple
   immediate, or a relocatable value.
 - ``n``: An integer constant -- *not* including relocatable values.
-- ``s``: An integer constant, but allowing *only* relocatable values.
+- ``s``: A symbol or label reference with a constant offset.
 - ``X``: Allows an operand of any kind, no constraint whatsoever. Typically
   useful to pass a label for an asm branch or call.
 
@@ -5283,6 +5283,7 @@ RISC-V:
 - ``f``: A 32- or 64-bit floating-point register (requires F or D extension).
 - ``r``: A 32- or 64-bit general-purpose register (depending on the platform
   ``XLEN``).
+- ``S``: Alias for ``s``.
 - ``vr``: A vector register. (requires V extension).
 - ``vm``: A vector register for masking operand. (requires V extension).
 

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -19188,6 +19188,7 @@ RISCVTargetLowering::getConstraintType(StringRef Constraint) const {
       return C_Immediate;
     case 'A':
       return C_Memory;
+    case 's':
     case 'S': // A symbolic address
       return C_Other;
     }
@@ -19449,13 +19450,7 @@ void RISCVTargetLowering::LowerAsmOperandForConstraint(
       }
       return;
     case 'S':
-      if (const auto *GA = dyn_cast<GlobalAddressSDNode>(Op)) {
-        Ops.push_back(DAG.getTargetGlobalAddress(GA->getGlobal(), SDLoc(Op),
-                                                 GA->getValueType(0)));
-      } else if (const auto *BA = dyn_cast<BlockAddressSDNode>(Op)) {
-        Ops.push_back(DAG.getTargetBlockAddress(BA->getBlockAddress(),
-                                                BA->getValueType(0)));
-      }
+      TargetLowering::LowerAsmOperandForConstraint(Op, "s", Ops, DAG);
       return;
     default:
       break;

llvm/test/CodeGen/RISCV/inline-asm-S-constraint.ll

@@ -0,0 +1,14 @@
+; RUN: not llc -mtriple=riscv64 < %s 2>&1 | FileCheck %s
+
+@a = external global [4 x i32], align 16
+
+; CHECK-COUNT-2: error: invalid operand for inline asm constraint 's'
+; CHECK-NOT:     error:
+define void @test(i64 %i) {
+entry:
+  %x = alloca i32, align 4
+  %ai = getelementptr inbounds [4 x i32], ptr @a, i64 0, i64 %i
+  call void asm sideeffect "", "s,~{dirflag},~{fpsr},~{flags}"(ptr %x)
+  call void asm sideeffect "", "s,~{dirflag},~{fpsr},~{flags}"(ptr %ai)
+  ret void
+}

llvm/test/CodeGen/RISCV/inline-asm-s-constraint-error.ll

@@ -0,0 +1,70 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv32 -relocation-model=static < %s | FileCheck %s --check-prefix=RV32
+; RUN: llc -mtriple=riscv64 -relocation-model=pic < %s | FileCheck %s --check-prefix=RV64
+
+@var = external dso_local global i32, align 4
+@a = external global [2 x [2 x i32]], align 4
+
+define dso_local void @test() {
+; RV32-LABEL: test:
+; RV32:       # %bb.0: # %entry
+; RV32-NEXT:    #APP
+; RV32-NEXT:    # var a+12 test
+; RV32-NEXT:    #NO_APP
+; RV32-NEXT:    #APP
+; RV32-NEXT:    # var a+12 test
+; RV32-NEXT:    #NO_APP
+; RV32-NEXT:    ret
+;
+; RV64-LABEL: test:
+; RV64:       # %bb.0: # %entry
+; RV64-NEXT:    #APP
+; RV64-NEXT:    # var a+12 .Ltest$local
+; RV64-NEXT:    #NO_APP
+; RV64-NEXT:    #APP
+; RV64-NEXT:    # var a+12 .Ltest$local
+; RV64-NEXT:    #NO_APP
+; RV64-NEXT:    ret
+entry:
+  call void asm sideeffect "// $0 $1 $2", "s,s,s,~{dirflag},~{fpsr},~{flags}"(ptr @var, ptr getelementptr inbounds ([2 x [2 x i32]], ptr @a, i64 0, i64 1, i64 1), ptr @test)
+
+  ;; Implement "S" as an alias for "s".
+  call void asm sideeffect "// $0 $1 $2", "S,S,S,~{dirflag},~{fpsr},~{flags}"(ptr @var, ptr getelementptr inbounds ([2 x [2 x i32]], ptr @a, i64 0, i64 1, i64 1), ptr @test)
+  ret void
+}
+
+; Function Attrs: nofree nosync nounwind readnone
+define dso_local ptr @test_label() {
+; RV32-LABEL: test_label:
+; RV32:       # %bb.0: # %entry
+; RV32-NEXT:  .Ltmp0: # Block address taken
+; RV32-NEXT:  # %bb.1: # %L1
+; RV32-NEXT:    #APP
+; RV32-NEXT:    # .Ltmp0
+; RV32-NEXT:    #NO_APP
+; RV32-NEXT:    #APP
+; RV32-NEXT:    lui a0, %hi(.Ltmp0)
+; RV32-NEXT:    addi a0, a0, %lo(.Ltmp0)
+; RV32-NEXT:    #NO_APP
+; RV32-NEXT:    ret
+;
+; RV64-LABEL: test_label:
+; RV64:       # %bb.0: # %entry
+; RV64-NEXT:  .Ltmp0: # Block address taken
+; RV64-NEXT:  # %bb.1: # %L1
+; RV64-NEXT:    #APP
+; RV64-NEXT:    # .Ltmp0
+; RV64-NEXT:    #NO_APP
+; RV64-NEXT:    #APP
+; RV64-NEXT:    lui a0, %hi(.Ltmp0)
+; RV64-NEXT:    addi a0, a0, %lo(.Ltmp0)
+; RV64-NEXT:    #NO_APP
+; RV64-NEXT:    ret
+entry:
+  br label %L1
+
+L1:
+  call void asm sideeffect "// $0", "s,~{dirflag},~{fpsr},~{flags}"(ptr blockaddress(@test_label, %L1))
+  %ret = tail call ptr asm "lui $0, %hi($1)\0Aaddi $0, $0, %lo($1)", "=r,S"(ptr blockaddress(@test_label, %L1))
+  ret ptr %ret
+}