Tree-optimization: Avoids ICE with niter Analysis and UBSAN.
The compiler now avoids an internal compiler error (ICE) when using niter analysis with UBSAN by preventing negation of the most negative signed integer.
This commit fixes an internal compiler error (ICE) that occurred when using the niter analysis (loop iteration analysis) in conjunction with UBSAN (Undefined Behavior Sanitizer). The issue arose when computing the absolute step of a loop by negating a signed step, which could result in negating the most negative signed integer. The fix converts the signed step to an unsigned value before negating it, thus avoiding the undefined behavior. A new testcase was added to verify the fix.
In Details
This commit addresses PR125025, an ICE in tree-ssa-loop-niter.cc. The functions number_of_iterations_ne and number_of_iterations_lt were attempting to negate a signed step, potentially leading to undefined behavior when negating the most negative signed integer. This commit changes the code to convert the step to unsigned before negating, mirroring existing practice elsewhere in the code. The associated testcase is gcc.dg/torture/pr125025.c.
For Context
Loop iteration analysis (niter analysis) is a compiler optimization technique that attempts to determine the number of times a loop will execute. This information can be used to optimize the loop and generate more efficient code. UBSAN (Undefined Behavior Sanitizer) is a tool that detects undefined behavior in C and C++ programs at runtime. This commit fixes a bug where the compiler would crash (ICE) when performing loop iteration analysis on code that also used UBSAN. The fix prevents the compiler from trying to negate the most negative signed integer, which is an undefined operation. This ensures that the compiler can correctly analyze loops and generate optimized code, even when UBSAN is enabled.