[Bug target/114659] gcc miscompiles a __builtin_memcpy on i386, leading to wrong results for SNaN
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659 --- Comment #9 from Bruno Haible --- (In reply to Andrew Pinski from comment #7) > Much more related to PR 56831 and PR 57484 rather than the other two ... Well, bug #56831 is more about function calls and the ABI, whereas this bug here and bug #58416 and bug #93271 are about the compiler picking a memory location which holds convert_snan_to_qnan(value) rather than a memory location which holds the original value.
[Bug target/114659] gcc miscompiles a __builtin_memcpy on i386, leading to wrong results for SNaN
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659 --- Comment #8 from Bruno Haible --- (In reply to Andrew Pinski from comment #6) > I doubt there is not much to be done here. I see it as an incorrect modelization of the x87 hardware, together with a missing distinction in the common expression elimination / aliasing analysis. In detail: * Incorrect modelization of the x87 hardware: The compiler seems to assume that flds MEM_LOCATION_1 fsts MEM_LOCATION_2 will result in MEM_LOCATION_2 having the same value as MEM_LOCATION_1. This is wrong; this is not how the x87 hardware behaves. The actual result is: *MEM_LOCATION_2 = convert_snan_to_qnan(*MEM_LOCATION_1). * In the common expression elimination / aliasing analysis, the compilers seems to keep track of a set of memory locations MEM_LOCATION_1, ..., MEM_LOCATION_n which have the same value. In fact, this set needs to be partitioned into two sets: a subset which contains the same value, and the complementary subset which contains convert_snan_to_qnan(value). In other words, each element of the set needs to be annotated with a bit that tells whether the value has been subject to the convert_snan_to_qnan.
[Bug target/114659] gcc miscompiles a __builtin_memcpy on i386, leading to wrong results for SNaN
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659 Andrew Pinski changed: What|Removed |Added See Also||https://gcc.gnu.org/bugzill ||a/show_bug.cgi?id=56831, ||https://gcc.gnu.org/bugzill ||a/show_bug.cgi?id=57484 --- Comment #7 from Andrew Pinski --- Much more related to PR 56831 and PR 57484 rather than the other two ...
[Bug target/114659] gcc miscompiles a __builtin_memcpy on i386, leading to wrong results for SNaN
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659 Andrew Pinski changed: What|Removed |Added Component|middle-end |target --- Comment #6 from Andrew Pinski --- I doubt there is not much to be done here. It is a x87 issue where we do the store of the float register stack register to the stack to get 32bits (or 64bit) version. And then load it into a GPR. float t = *x; float t1 = *y; __builtin_memcpy (, , sizeof (float)); __builtin_memcpy (, , sizeof (float)); Produces exactly the same issue.
