On Wed, Nov 1, 2023 at 12:30 PM Giuseppe Tagliavini via Gcc <gcc@gcc.gnu.org> wrote: > > I found an unexpected issue working with an experimental target (available > here: https://github.com/EEESlab/tricore-gcc), but I was able to reproduce it > on mainstream architectures. For the sake of clarity and reproducibility, I > always refer to upstream code in the rest of the discussion. > > Consider this simple test: > > #include <stdio.h> > int f(unsigned int a) { > unsigned int res = 8*sizeof(unsigned int) - __builtin_clz(a); > if(res>0) printf("test passed\n"); > return res-1; > } > > I tested this code on GCC 9 and GCC 11 branches, obtaining the expected > result from GCC 9 and the wrong one from GCC 11. In GCC 11 and newer > versions, the condition check is removed by a gimple-level optimization (I > will provide details later), and the printf is always invoked at the assembly > level with no branch. > > According to the GCC manual, __builtin_clz "returns the number of leading > 0-bits in x, starting at the most significant bit position. If x is 0, the > result is undefined." However, it is possible to define a > CLZ_DEFINED_VALUE_AT_ZERO in the architecture backend to specify a defined > behavior for this case. For instance, this has been done for SPARC and > AARCH64 architectures. Compiling my test with SPARC GCC 13.2.0 with the -O3 > flag on CompilerExplorer I got this assembly:
Note the semantic of __builtin_clz is _not_ altered by CLZ_DEFINED_VALUE_AT_ZERO, the behavior of __builtin_clz (x) is that is has undefined result for x == 0. CLZ_DEFINED_VALUE_AT_ZERO is only used to optimize code generation when the user writes say x == 0 ? 0 : __builtin_clz (x) Richard. > .LC0: > .asciz "test" > f: > save %sp, -96, %sp > call __clzsi2, 0 > mov %i0, %o0 > mov %o0, %i0 > sethi %hi(.LC0), %o0 > call printf, 0 > or %o0, %lo(.LC0), %o0 > mov 31, %g1 > return %i7+8 > sub %g1, %o0, %o0 > > After some investigation, I found this optimization derives from the results > of the value range propagation analysis: > https://github.com/gcc-mirror/gcc/blob/master/gcc/gimple-range-op.cc#L917 > In this code, I do not understand why CLZ_DEFINED_VALUE_AT_ZERO is verified > only if the function call is tagged as internal. A gimple call is tagged as > internal at creation time only when there is no associated function > declaration (see > https://github.com/gcc-mirror/gcc/blob/master/gcc/gimple.cc#L371), which is > not the case for the builtins. From my point of view, this condition prevents > the computation of the correct upper bound for this case, resulting in a > wrong result from the VRP analysis. > > Before considering this behavior as a bug, I prefer to ask the community to > understand if there is any aspect I have missed in my reasoning.
