[llvm] [Analysis] Ensure use of strict fp exceptions in ConstantFolding (PR #136139)

[Lucas Duarte Prates via llvm-commits]

https://github.com/pratlucas created https://github.com/llvm/llvm-project/pull/136139

To perform constant folding in math operations, the implementation of
the ConstantFolding Analysis relies on the use of the math functions
from the host's libm. In particular, it relies on checking the value of
errno and IEEE exceptions to determine when an operation is safe to be
constant-folded.

On some platforms, such as BSD or Darwin, math library functions don't
set errno, so the ConstantFolding check depends only on the value of
IEEE exceptions. As the FP exception behaviour is set to `ignore` by
default, the compiler can perform optimisations that would get in the
way of such checks being performed correctly.

This patch sets the FP exception behaviour to `strict` when compiling
the `ConstantFolding.cpp` source file, ensuring the value of IEEE
exceptions can be reliably used by its implementation.


>From 8cc14b4dbc8340b731373d9b70208e1da77b16e0 Mon Sep 17 00:00:00 2001
From: Lucas Prates <lucas.prates at arm.com>
Date: Wed, 16 Apr 2025 16:29:58 +0100
Subject: [PATCH] [Analysis] Ensure use of strict fp exceptions in
 ConstantFolding

To perform constant folding in math operations, the implementation of
the ConstantFolding Analysis relies on the use of the math functions
from the host's libm. In particular, it relies on checking the value of
errno and IEEE exceptions to determine when an operation is safe to be
constant-folded.

On some platforms, such as BSD or Darwin, math library functions don't
set errno, so the ConstantFolding check depends only on the value of
IEEE exceptions. As the FP exception behaviour is set to `ignore` by
default, the compiler can perform optimisations that would get in the
way of such checks being performed correctly.

This patch sets the FP exception behaviour to `strict` when compiling
the `ConstantFolding.cpp` source file, ensuring the value of IEEE
exceptions can be reliably used by its implementation.
---
 llvm/lib/Analysis/CMakeLists.txt | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/llvm/lib/Analysis/CMakeLists.txt b/llvm/lib/Analysis/CMakeLists.txt
index fbf3b587d6bd2..a37f088359c64 100644
--- a/llvm/lib/Analysis/CMakeLists.txt
+++ b/llvm/lib/Analysis/CMakeLists.txt
@@ -23,6 +23,17 @@ if (DEFINED LLVM_HAVE_TF_AOT OR LLVM_HAVE_TFLITE)
   endif()
 endif()
 
+# The implementaiton of ConstantFolding.cpp relies on the use of math functions
+# from the host. In particular, it relies on the detection of floating point
+# exceptions originating from such math functions to prevent invalid cases
+# from being constant folded. Therefore, we must ensure that fp exceptions are
+# handled correctly.
+if (MSVC)
+  set_source_files_properties(ConstantFolding.cpp PROPERTIES COMPILE_OPTIONS "/fp:except")
+else()
+  set_source_files_properties(ConstantFolding.cpp PROPERTIES COMPILE_OPTIONS "-ffp-exception-behavior=strict")
+endif()
+
 add_llvm_component_library(LLVMAnalysis
   AliasAnalysis.cpp
   AliasAnalysisEvaluator.cpp