On 26 Oct 2006 22:02:04 -0700, Ian Lance Taylor <[EMAIL PROTECTED]> wrote:
"Rohit Arul Raj" <[EMAIL PROTECTED]> writes:> This small bit of code worked fine with all optimization except Os. > > unsigned int n = 30; > void x () > { > unsigned int h; > h = n <= 30; // Line 1 > if (h) > p = 1; > else > p = 0; > } > > when we tried to debug the emitted RTL instruction for Os, it was > found that RTL instruction for Line #1 (Compare and gtu) were not at > all emitted. i.e. there is no reference with respect to "h or n". For > the same optimization Os, if we declare the identifier "h" as global, > it generates the instructions properly. That small bit of code won't compile, since 'p' is undeclared. If 'p' is a global variable, then this certainly seems to be a bug. But you should be aware that -Os changes inlining behaviour, and different inlining may cause different behaviour here depending on how 'p' is used.
p is a global variable. But the problem is not with p, but with codegeneration for h, n.
> While checking the Dumps of RTL, the above mentioned code for "h, n" > was present in the file 20020611-1.c.25.cse2 but not in > 20020611-1.c.26.life1. > > 1. Is the file 20020611-1.c.25.cse2 input to life1 optimization pass? It is a dump of what the intermediate representation looks like after the cse2 pass.
In the RTL dump after cse2 pass, i do have the code for h, n. But in the RTL dump after life analysis, that part of code is missing. Is this output of cse2 pass used for life analysis?
> 2. What does .life1 Life analysis pass do ? It does flow analysis: it records which pseudo-registers are live at all parts of the program. It also does a number of minor random cleanups.
> 3. What are the probable causes for the elimination of RTL code's > (Compare & gtu) between the above mentioned passes? The probable cause is that 'p' appears to be unused, and the assignment to 'p' is dead.
since p is a global variable, it can be used in other functions. Any other causes? 4. If i disable gcse optimization pass, (i.e. -Os -fno-gcse), the code is generated properly for h , n and the program works fine (it failed for -Os). How does gcse affect code generation at .life1 pass. Regards, Rohit
