[libc] Make FPUtils' rounding_mode.h functions constexpr. #149167
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@llvm/pr-subscribers-libc Author: None (lntue) ChangesFull diff: https://github.com/llvm/llvm-project/pull/149167.diff 2 Files Affected:
diff --git a/libc/src/__support/FPUtil/CMakeLists.txt b/libc/src/__support/FPUtil/CMakeLists.txt
index cc941f23135a6..764cb33a83c0e 100644
--- a/libc/src/__support/FPUtil/CMakeLists.txt
+++ b/libc/src/__support/FPUtil/CMakeLists.txt
@@ -16,6 +16,7 @@ add_header_library(
rounding_mode.h
DEPENDS
libc.hdr.fenv_macros
+ libc.src.__support.CPP.type_traits
libc.src.__support.macros.attributes
libc.src.__support.macros.properties.architectures
libc.src.__support.macros.sanitizer
diff --git a/libc/src/__support/FPUtil/rounding_mode.h b/libc/src/__support/FPUtil/rounding_mode.h
index bc66d09b94160..4ee0a0b0490fc 100644
--- a/libc/src/__support/FPUtil/rounding_mode.h
+++ b/libc/src/__support/FPUtil/rounding_mode.h
@@ -10,6 +10,7 @@
#define LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
#include "hdr/fenv_macros.h"
+#include "src/__support/CPP/type_traits.h" // is_constant_evaluated
#include "src/__support/macros/attributes.h" // LIBC_INLINE
#include "src/__support/macros/config.h"
@@ -20,18 +21,26 @@ namespace fputil {
// Using the following observation:
// 1.0f + 2^-25 = 1.0f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
// = 0x1.000002f for FE_UPWARD.
-LIBC_INLINE bool fenv_is_round_up() {
- volatile float x = 0x1.0p-25f;
- return (1.0f + x != 1.0f);
+LIBC_INLINE static constexpr bool fenv_is_round_up() {
+ if (cpp::is_constant_evaluated()) {
+ return false;
+ } else {
+ volatile float x = 0x1.0p-25f;
+ return (1.0f + x != 1.0f);
+ }
}
// Quick free-standing test whether fegetround() == FE_DOWNWARD.
// Using the following observation:
// -1.0f - 2^-25 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
// = -0x1.000002f for FE_DOWNWARD.
-LIBC_INLINE bool fenv_is_round_down() {
- volatile float x = 0x1.0p-25f;
- return (-1.0f - x != -1.0f);
+LIBC_INLINE static constexpr bool fenv_is_round_down() {
+ if (cpp::is_constant_evaluated()) {
+ return false;
+ } else {
+ volatile float x = 0x1.0p-25f;
+ return (-1.0f - x != -1.0f);
+ }
}
// Quick free-standing test whether fegetround() == FE_TONEAREST.
@@ -40,10 +49,14 @@ LIBC_INLINE bool fenv_is_round_down() {
// = 0x1.100002p0f for FE_UPWARD,
// 1.5f - 2^-24 = 1.5f for FE_TONEAREST, FE_UPWARD
// = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
-LIBC_INLINE bool fenv_is_round_to_nearest() {
- static volatile float x = 0x1.0p-24f;
- float y = x;
- return (1.5f + y == 1.5f - y);
+LIBC_INLINE static constexpr bool fenv_is_round_to_nearest() {
+ if (cpp::is_constant_evaluated()) {
+ return true;
+ } else {
+ volatile float x = 0x1.0p-24f;
+ float y = 1.5f + x;
+ return (y == 1.5f - x);
+ }
}
// Quick free-standing test whether fegetround() == FE_TOWARDZERO.
@@ -56,23 +69,31 @@ LIBC_INLINE bool fenv_is_round_to_nearest() {
// (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
// = 2^-22 for FE_TONEAREST, FE_UPWARD
// = 0 for FE_DOWNWARD
-LIBC_INLINE bool fenv_is_round_to_zero() {
- static volatile float x = 0x1.0p-24f;
- float y = x;
- return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
+LIBC_INLINE static constexpr bool fenv_is_round_to_zero() {
+ if (cpp::is_constant_evaluated()) {
+ return false;
+ } else {
+ volatile float x = 0x1.0p-24f;
+ volatile float y = 0x1.000002p0f + x;
+ return (y + (-1.0f - x) == 0x1.0p-23f);
+ }
}
// Quick free standing get rounding mode based on the above observations.
-LIBC_INLINE int quick_get_round() {
- static volatile float x = 0x1.0p-24f;
- float y = x;
- float z = (0x1.000002p0f + y) + (-1.0f - y);
+LIBC_INLINE static constexpr int quick_get_round() {
+ if (cpp::is_constant_evaluated()) {
+ return FE_TONEAREST;
+ } else {
+ volatile float x = 0x1.0p-24f;
+ volatile float y = 0x1.000002p0f + x;
+ float z = y + (-1.0f - x);
- if (z == 0.0f)
- return FE_DOWNWARD;
- if (z == 0x1.0p-23f)
- return FE_TOWARDZERO;
- return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
+ if (z == 0.0f)
+ return FE_DOWNWARD;
+ if (z == 0x1.0p-23f)
+ return FE_TOWARDZERO;
+ return (2.0f + x == 2.0f) ? FE_TONEAREST : FE_UPWARD;
+ }
}
} // namespace fputil
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overmighty
approved these changes
Jul 23, 2025
Comment on lines
+25
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+27
| if (cpp::is_constant_evaluated()) { | ||
| return false; | ||
| } else { |
There was a problem hiding this comment.
Nit: if if constexpr is not needed then the else after return should be removed as per the LLVM Coding Standards. Same for the other functions changed.
There was a problem hiding this comment.
I think the codegen semantic of is_constant_evaluated is similar to if constexpr / if consteval, so the else clause will make sure that the non-constexpr branch is not seen in the constexpr context.
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/10/builds/10025 Here is the relevant piece of the build log for the reference |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/39088 Here is the relevant piece of the build log for the reference |
|
Hi there, this PR caused build error in our bot. Could you please take a look? Thanks! bot: https://lab.llvm.org/buildbot/#/builders/10/builds/10025 |
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