From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eli Zaretskii To: Mark Kettenis Cc: gdb-patches@sources.redhat.com Subject: Re: [PATCH] Fix returning floating points values for x86 Date: Thu, 12 Jul 2001 00:00:00 -0000 Message-id: References: <200107112028.f6BKSOS26755@delius.kettenis.local> X-SW-Source: 2001-07/msg00281.html On Wed, 11 Jul 2001, Mark Kettenis wrote: > The problem the patch solves is returning from a > function with the GDB `return' command, or more specifically returning > from a function that has a floating point return value. You mean, the "return EXPRESSION" command, where EXPRESSION is an FP expression, right? > If we suppose that the FPU starts out in a freshly initialized state, > with all registers empty and TOP set to 0, this means that we'll > always end up with storing return values in the hardware register 7, > i.e. with TOP set to 7. So it makes sense to reset the FPU to that > state in i386_store_return_value. What value we choose for TOP is in > principle irrelevant since FP0_REGNO always refers to %st(0). But we > must mark the right register as valid (and all others as empty) in the > tag word, and here the value of TOP does matter. If we don't mark the > right register as valid, the caller of the function won't be able to > pop the return value from the stack, and if we don't mark the other > registers as empty we might trigger an unwanted stack overflow. Okay, I think I understand now: you are simulating the state of the FPU at function return point. Perhaps this should be added to the comments. Thanks.