http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56925
Bug #: 56925
Summary: SRA should take into account likelihood of statements
being executed
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: josh.m.con...@gmail.com
In the following code:
struct stype {
unsigned int pad:4;
unsigned int val:4;
};
void bar (void);
void baz (void);
int x, y;
unsigned int foo (struct stype input)
{
if (__builtin_expect (x, 0))
return input.val;
if (__builtin_expect (y, 0))
return input.val + 1;
return 0;
}
When compiled with -O2, SRA moves the read of input.val to the top of the
function:
;; Function foo (foo, funcdef_no=0, decl_uid=4988, cgraph_uid=0)
Candidate (4987): input
Rejected (4999): not aggregate: y.1
Rejected (4993): not aggregate: x.0
Created a replacement for input offset: 4, size: 4: input$val
...
<bb 2>:
input$val_14 = input.val;
x.0_3 = x;
_4 = __builtin_expect (x.0_3, 0);
if (_4 != 0)
goto <bb 3>;
else
goto <bb 4>;
...
Which means that the critical path for this function now executes an extra
instruction.
It would be nice if SRA would take into account the likelihood of statement
execution when deciding whether to apply the transformation. We currently
verify that there are at least two reads -- perhaps we should check that there
are at least two reads that are likely to occur.
This can be seen in sub-optimal codegen for ARM, where a bitfield extract
(ubfx) is moved out of unlikely code into the critical path:
foo:
movw r3, #:lower16:x
ubfx r2, r0, #4, #4
movt r3, #:upper16:x
ldr r3, [r3]
cmp r3, #0
bne .L6
...
