Red Hat ENTERPRISE LINUX 3 - USING GCC Using Instructions page 57

Chapter 4. GCC Command Options
The default is
-fbranch-count-reg
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function
contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output
may be confused by the optimizations performed when this option is not used.
The default is
-ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero
into BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to
the data section. E.g., so that the resulting executable can find the beginning of that section and/or
make assumptions based on that.
The default is
-fzero-initialized-in-bss
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another
comparison subsumed by the first is found. If so, the first branch is redirected to either the des-
tination of the second branch or a point immediately following it, depending on whether the
condition is known to be true or false.
Enabled at levels
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the
jump is not reached by any other path. For example, when CSE encounters an
an clause, CSE will follow the jump when the condition tested is false.
else
Enabled at levels
-fcse-skip-blocks
This is similar to
skip over blocks. When CSE encounters a simple
-fcse-skip-blocks
Enabled at levels
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels
, , .
-O2 -O3 -Os
, , , .
-O -O2 -O3 -Os
, , .
-O2 -O3 -Os
-fcse-follow-jumps
causes CSE to follow the jump around the body of the
, , .
-O2 -O3 -Os
, , .
-O2 -O3 -Os
, , .
-O2 -O3 -Os
, enabled when
-fstrength-reduce
.
, but causes CSE to follow jumps which conditionally
if
is enabled.
statement with
if
statement with no else clause,
.
if
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