http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56894
Bug #: 56894
Summary: performance regression in gcc 4.7.x due to a +=
operation
Classification: Unclassified
Product: gcc
Version: 4.7.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: laurent.alfo...@st.com
Hello,
I have identified a big performance regression between 4.6 and 4.7. (I have
enclosed the pathological test).
After investigation, it is because of the += statement applied on 2 signed
chars.
- It is now type-promoted to "int" when it is written "result += foo()".
(since 4.7)
- it is type promoted to "unsigned char" when it is written "result = result
+ foo()".
The "char->int->char" cast is blocking the sccp constant propagation
optimization.
More precisely, in gcc 4.6, the "result" variable is seen as a loop_invariant
(no_evolution_in_loop_p):
<nop_expr 0xf7fbb8a4
type <integer_type 0xf7ee4180 char sizes-gimplified public string-flag
type_6 QI [...]>
arg 0 <polynomial_chrec 0xf7fb96e4
type <integer_type 0xf7ee4120 unsigned char public unsigned string-flag
QI [...]>
arg 0 <integer_cst 0xf7eccac0 constant 2>
arg 1 <integer_cst 0xf7ecc120 constant 0>
arg 2 <integer_cst 0xf7f88e20 constant 1>>>
In gcc 4.7, the result variable is not seen as a loop_invariant due to the cast
to int:
type <integer_type 0xf7de32a0 char sizes-gimplified public string-flag
type_6 QI[...]>
arg 0 <polynomial_chrec 0xf7ecd180
type <integer_type 0xf7de33c0 int sizes-gimplified public type_6
SI[...]>
arg 0 <integer_cst 0xf7dcbe70 constant 2>
arg 1 <integer_cst 0xf7dcba80 constant 0>
arg 2 <integer_cst 0xf7dcba9c constant 1>>>
I don't know how to change the chrec detection to recover the constant
propagation in gcc 4.7.
Thanks,
Laurent
