[libc] Skip x87 floating point register and only update mxcsr for x86_64 targets when raising exceptions inside math functions.

[lntue]

Updating x87 floating point register significantly affect the performance of the functions.
All the floating point exception reads will merge the results from both mxcsr and x87 registers anyway.

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (lntue)

Changes

Updating x87 floating point register significantly affect the performance of the functions.
All the floating point exception reads will merge the results from both mxcsr and x87 registers anyway.

---

Full diff: https://github.com/llvm/llvm-project/pull/144951.diff

2 Files Affected:

• (modified) libc/src/__support/FPUtil/FEnvImpl.h (+5)
• (modified) libc/src/__support/FPUtil/x86_64/FEnvImpl.h (+8-5)

diff --git a/libc/src/__support/FPUtil/FEnvImpl.h b/libc/src/__support/FPUtil/FEnvImpl.h
index ba145a3da45cc..76910880eb810 100644
--- a/libc/src/__support/FPUtil/FEnvImpl.h
+++ b/libc/src/__support/FPUtil/FEnvImpl.h
@@ -91,7 +91,12 @@ LIBC_INLINE static int set_except_if_required([[maybe_unused]] int excepts) {
 LIBC_INLINE static int raise_except_if_required([[maybe_unused]] int excepts) {
 #ifndef LIBC_MATH_HAS_NO_EXCEPT
   if (math_errhandling & MATH_ERREXCEPT)
+#ifdef LIBC_TARGET_ARCH_IS_X86_64
+    return raise_except</*SKIP_X87_FPU*/ true>(excepts);
+#else  // !LIBC_TARGET_ARCH_IS_X86
     return raise_except(excepts);
+#endif // LIBC_TARGET_ARCH_IS_X86
+
 #endif // LIBC_MATH_HAS_NO_EXCEPT
   return 0;
 }
diff --git a/libc/src/__support/FPUtil/x86_64/FEnvImpl.h b/libc/src/__support/FPUtil/x86_64/FEnvImpl.h
index b77178ea69ea0..560727c22978e 100644
--- a/libc/src/__support/FPUtil/x86_64/FEnvImpl.h
+++ b/libc/src/__support/FPUtil/x86_64/FEnvImpl.h
@@ -239,7 +239,7 @@ LIBC_INLINE int set_except(int excepts) {
   return 0;
 }
 
-LIBC_INLINE int raise_except(int excepts) {
+template <bool SKIP_X87_FPU = false> LIBC_INLINE int raise_except(int excepts) {
   uint16_t status_value = internal::get_status_value_for_except(excepts);
 
   // We set the status flag for exception one at a time and call the
@@ -256,13 +256,16 @@ LIBC_INLINE int raise_except(int excepts) {
   // when raising the next exception.
 
   auto raise_helper = [](uint16_t singleExceptFlag) {
-    internal::X87StateDescriptor state;
+    if constexpr (!SKIP_X87_FPU) {
+      internal::X87StateDescriptor state;
+      internal::get_x87_state_descriptor(state);
+      state.status_word |= singleExceptFlag;
+      internal::write_x87_state_descriptor(state);
+    }
+
     uint32_t mxcsr = 0;
-    internal::get_x87_state_descriptor(state);
     mxcsr = internal::get_mxcsr();
-    state.status_word |= singleExceptFlag;
     mxcsr |= singleExceptFlag;
-    internal::write_x87_state_descriptor(state);
     internal::write_mxcsr(mxcsr);
     internal::fwait();
   };