[clang] [X86] Allow using the lzcnt intrinsics for non-LZCNT targets (PR #128284)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-clang

Author: Phoebe Wang (phoebewang)

<details>
<summary>Changes</summary>

Similar to D14748, we can relax lzcnt intrinsics too, especially with improved BSR lowering by #<!-- -->123623

---
Full diff: https://github.com/llvm/llvm-project/pull/128284.diff


4 Files Affected:

- (modified) clang/include/clang/Basic/BuiltinsX86.td (+2-5) 
- (modified) clang/include/clang/Basic/BuiltinsX86_64.td (+1-4) 
- (modified) clang/lib/Headers/lzcntintrin.h (+2-3) 
- (modified) clang/test/CodeGen/X86/lzcnt-builtins.c (+3-3) 


``````````diff
diff --git a/clang/include/clang/Basic/BuiltinsX86.td b/clang/include/clang/Basic/BuiltinsX86.td
index 572ac7235be02..e2252620bab56 100644
--- a/clang/include/clang/Basic/BuiltinsX86.td
+++ b/clang/include/clang/Basic/BuiltinsX86.td
@@ -832,11 +832,6 @@ let Features = "rdseed", Attributes = [NoThrow] in {
   def rdseed32_step : X86Builtin<"unsigned int(unsigned int *)">;
 }
 
-let Features = "lzcnt", Attributes = [NoThrow, Const, Constexpr] in {
-  def lzcnt_u16 : X86Builtin<"unsigned short(unsigned short)">;
-  def lzcnt_u32 : X86Builtin<"unsigned int(unsigned int)">;
-}
-
 let Features = "bmi", Attributes = [NoThrow, Const, Constexpr] in {
   def bextr_u32 : X86Builtin<"unsigned int(unsigned int, unsigned int)">;
 }
@@ -844,6 +839,8 @@ let Features = "bmi", Attributes = [NoThrow, Const, Constexpr] in {
 let Attributes = [NoThrow, Const, Constexpr] in {
   def tzcnt_u16 : X86Builtin<"unsigned short(unsigned short)">;
   def tzcnt_u32 : X86Builtin<"unsigned int(unsigned int)">;
+  def lzcnt_u16 : X86Builtin<"unsigned short(unsigned short)">;
+  def lzcnt_u32 : X86Builtin<"unsigned int(unsigned int)">;
 }
 
 let Features = "bmi2", Attributes = [NoThrow, Const, Constexpr] in {
diff --git a/clang/include/clang/Basic/BuiltinsX86_64.td b/clang/include/clang/Basic/BuiltinsX86_64.td
index 4958265298d1b..f2b35874e3876 100644
--- a/clang/include/clang/Basic/BuiltinsX86_64.td
+++ b/clang/include/clang/Basic/BuiltinsX86_64.td
@@ -126,16 +126,13 @@ let Features = "rdseed", Attributes = [NoThrow] in {
   def rdseed64_step : X86Builtin<"unsigned int(unsigned long long int *)">;
 }
 
-let Features = "lzcnt", Attributes = [NoThrow, Const, Constexpr] in {
-  def lzcnt_u64 : X86Builtin<"unsigned long long int(unsigned long long int)">;
-}
-
 let Features = "bmi", Attributes = [NoThrow, Const, Constexpr] in {
   def bextr_u64 : X86Builtin<"unsigned long long int(unsigned long long int, unsigned long long int)">;
 }
 
 let Attributes = [NoThrow, Const, Constexpr] in {
   def tzcnt_u64 : X86Builtin<"unsigned long long int(unsigned long long int)">;
+  def lzcnt_u64 : X86Builtin<"unsigned long long int(unsigned long long int)">;
 }
 
 let Features = "bmi2", Attributes = [NoThrow, Const, Constexpr] in {
diff --git a/clang/lib/Headers/lzcntintrin.h b/clang/lib/Headers/lzcntintrin.h
index 27509021ec258..8d2d39f094aa0 100644
--- a/clang/lib/Headers/lzcntintrin.h
+++ b/clang/lib/Headers/lzcntintrin.h
@@ -17,10 +17,9 @@
 /* Define the default attributes for the functions in this file. */
 #if defined(__cplusplus) && (__cplusplus >= 201103L)
 #define __DEFAULT_FN_ATTRS                                                     \
-  __attribute__((__always_inline__, __nodebug__, __target__("lzcnt"))) constexpr
+  __attribute__((__always_inline__, __nodebug__)) constexpr
 #else
-#define __DEFAULT_FN_ATTRS                                                     \
-  __attribute__((__always_inline__, __nodebug__, __target__("lzcnt")))
+#define __DEFAULT_FN_ATTRS __attribute__((__always_inline__, __nodebug__))
 #endif
 
 #ifndef _MSC_VER
diff --git a/clang/test/CodeGen/X86/lzcnt-builtins.c b/clang/test/CodeGen/X86/lzcnt-builtins.c
index 18ced89fc79b1..212155f123adc 100644
--- a/clang/test/CodeGen/X86/lzcnt-builtins.c
+++ b/clang/test/CodeGen/X86/lzcnt-builtins.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -x c -ffreestanding %s -triple=x86_64-apple-darwin -target-feature +lzcnt -emit-llvm -o - | FileCheck %s
-// RUN: %clang_cc1 -x c++ -std=c++11 -ffreestanding %s -triple=x86_64-apple-darwin -target-feature +lzcnt -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -x c -ffreestanding %s -triple=x86_64-apple-darwin -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -x c++ -std=c++11 -ffreestanding %s -triple=x86_64-apple-darwin -emit-llvm -o - | FileCheck %s
 
 
 #include <immintrin.h>
@@ -56,4 +56,4 @@ char lzcntu32_2[_lzcnt_u32(0x00000010) == 27 ? 1 : -1];
 char lzcntu64_0[_lzcnt_u64(0x0000000000000000ULL) == 64 ? 1 : -1];
 char lzcntu64_1[_lzcnt_u64(0x8000000000000000ULL) ==  0 ? 1 : -1];
 char lzcntu64_2[_lzcnt_u64(0x0000000100000000ULL) == 31 ? 1 : -1];
-#endif
\ No newline at end of file
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/128284