https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105477
--- Comment #2 from Christoph Müllner <christophm30 at gmail dot com> ---
I've analysed this issue a bit more and want to share my observations.
I mention commit 3a7ba8fd here again as trigger of this issue, but not
as the underlying issue (which I did not fully understand so far).
When looking into the dump files, the input to the sink2 pass (output of
forwprop4) is:
long int test (long int a, long int b, long int c)
{
long int iftmp.0_1;
<bb 2> [local count: 1073741824]:
if (c_2(D) == 0)
goto <bb 4>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 536870912]:
<bb 4> [local count: 1073741824]:
# iftmp.0_1 = PHI <a_4(D)(2), b_3(D)(3)>
return iftmp.0_1;
}
Before commit 3a7ba8fd this did not get further changed up to the expand pass.
Since commit 3a7ba8fd the sink2 pass transforms into the following:
long int test (long int a, long int b, long int c)
{
long int iftmp.0_1;
<bb 2> [local count: 1073741824]:
if (c_2(D) == 0)
goto <bb 3>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 5> [local count: 536870912]:
goto <bb 4>; [100.00%]
<bb 3> [local count: 536870912]:
<bb 4> [local count: 1073741824]:
# iftmp.0_1 = PHI <a_4(D)(3), b_3(D)(5)>
return iftmp.0_1;
}
This has an impact on
* the expansion pass (for obvious reasons)
* the output of the combiner pass
* the result of the reload pass
Let's start with the behaviour before change 3a7ba8fd.
The expander generated the following output:
(insn 2 7 3 2 (set (reg/v:DI 73 [ a ])
(reg:DI 10 a0 [ a ])) "pr105477.c":2:1 -1
(nil))
(insn 3 2 4 2 (set (reg/v:DI 74 [ b ])
(reg:DI 11 a1 [ b ])) "pr105477.c":2:1 -1
(nil))
(insn 4 3 5 2 (set (reg/v:DI 75 [ c ])
(reg:DI 12 a2 [ c ])) "pr105477.c":2:1 -1
(nil))
(jump_insn 9 5 10 2 (set (pc)
(if_then_else (eq (reg/v:DI 75 [ c ])
(const_int 0 [0]))
(label_ref 11)
(pc))) "pr105477.c":3:17 -1
(int_list:REG_BR_PROB 536870916 (nil))
-> 11)
(insn 6 10 11 4 (set (reg/v:DI 73 [ a ])
(reg/v:DI 74 [ b ])) "pr105477.c":3:17 -1
(nil))
(code_label 11 6 12 5 2 (nil) [1 uses])
(insn 13 12 17 5 (set (reg:DI 72 [ <retval> ])
(reg/v:DI 73 [ a ])) "pr105477.c":3:17 -1
(nil))
(insn 17 13 18 5 (set (reg/i:DI 10 a0)
(reg:DI 72 [ <retval> ])) "pr105477.c":4:1 -1
(nil))
(insn 18 17 0 5 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
The combiner then converted to:
(insn 30 7 2 2 (set (reg:DI 77)
(reg:DI 10 a0 [ a ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 10 a0 [ a ])
(nil)))
(insn 2 30 31 2 (set (reg/v:DI 73 [ a ])
(reg:DI 77)) "pr105477.c":2:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg:DI 77)
(nil)))
(insn 31 2 3 2 (set (reg:DI 78)
(reg:DI 11 a1 [ b ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 11 a1 [ b ])
(nil)))
(insn 3 31 32 2 (set (reg/v:DI 74 [ b ])
(reg:DI 78)) "pr105477.c":2:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg:DI 78)
(nil)))
(insn 32 3 4 2 (set (reg:DI 79)
(reg:DI 12 a2 [ c ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 12 a2 [ c ])
(nil)))
(jump_insn 9 5 10 2 (set (pc)
(if_then_else (eq (reg:DI 79)
(const_int 0 [0]))
(label_ref:DI 11)
(pc))) "pr105477.c":3:17 182 {*branchdi}
(expr_list:REG_DEAD (reg:DI 79)
(int_list:REG_BR_PROB 536870916 (nil)))
-> 11)
(insn 6 10 11 3 (set (reg/v:DI 73 [ a ])
(reg/v:DI 74 [ b ])) "pr105477.c":3:17 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg/v:DI 74 [ b ])
(nil)))
(code_label 11 6 12 4 2 (nil) [1 uses])
(insn 17 12 18 4 (set (reg/i:DI 10 a0)
(reg/v:DI 73 [ a ])) "pr105477.c":4:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg/v:DI 73 [ a ])
(nil)))
(insn 18 17 0 4 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
This could be simplified by the reload pass to:
(jump_insn 9 5 10 2 (set (pc)
(if_then_else (eq (reg:DI 12 a2 [79])
(const_int 0 [0]))
(label_ref:DI 11)
(pc))) "pr105477.c":3:17 182 {*branchdi}
(int_list:REG_BR_PROB 536870916 (nil))
-> 11)
(insn 6 10 11 3 (set (reg/v:DI 10 a0 [orig:73 a ] [73])
(reg/v:DI 11 a1 [orig:74 b ] [74])) "pr105477.c":3:17 135
{*movdi_64bit}
(nil))
(code_label 11 6 12 4 2 (nil) [1 uses])
(insn 18 12 36 4 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
The resulting assembly did not contain the two useless move instructions.
Since commit 3a7ba8fd the expanded code looks like this:
(insn 2 7 3 2 (set (reg/v:DI 73 [ a ])
(reg:DI 10 a0 [ a ])) "pr105477.c":2:1 -1
(nil))
(insn 3 2 4 2 (set (reg/v:DI 74 [ b ])
(reg:DI 11 a1 [ b ])) "pr105477.c":2:1 -1
(nil))
(insn 4 3 5 2 (set (reg/v:DI 75 [ c ])
(reg:DI 12 a2 [ c ])) "pr105477.c":2:1 -1
(nil))
(jump_insn 9 5 10 2 (set (pc)
(if_then_else (ne (reg/v:DI 75 [ c ])
(const_int 0 [0]))
(label_ref 11)
(pc))) "pr105477.c":3:17 -1
(int_list:REG_BR_PROB 536870916 (nil))
-> 11)
(insn 6 10 11 4 (set (reg/v:DI 74 [ b ])
(reg/v:DI 73 [ a ])) "pr105477.c":3:17 -1
(nil))
(code_label 11 6 12 5 2 (nil) [1 uses])
(insn 13 12 17 5 (set (reg:DI 72 [ <retval> ])
(reg/v:DI 74 [ b ])) "pr105477.c":3:17 -1
(nil))
(insn 17 13 18 5 (set (reg/i:DI 10 a0)
(reg:DI 72 [ <retval> ])) "pr105477.c":4:1 -1
(nil))
(insn 18 17 0 5 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
The code remains like this and gets changed by the combiner later on to:
(insn 24 7 2 2 (set (reg:DI 77)
(reg:DI 10 a0 [ a ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 10 a0 [ a ])
(nil)))
(insn 2 24 25 2 (set (reg/v:DI 73 [ a ])
(reg:DI 77)) "pr105477.c":2:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg:DI 77)
(nil)))
(insn 25 2 3 2 (set (reg:DI 78)
(reg:DI 11 a1 [ b ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 11 a1 [ b ])
(nil)))
(insn 3 25 26 2 (set (reg/v:DI 74 [ b ])
(reg:DI 78)) "pr105477.c":2:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg:DI 78)
(nil)))
(insn 26 3 4 2 (set (reg:DI 79)
(reg:DI 12 a2 [ c ])) "pr105477.c":2:1 -1
(expr_list:REG_DEAD (reg:DI 12 a2 [ c ])
(nil)))
(jump_insn 9 5 10 2 (set (pc)
(if_then_else (ne (reg:DI 79)
(const_int 0 [0]))
(label_ref 11)
(pc))) "pr105477.c":3:17 182 {*branchdi}
(expr_list:REG_DEAD (reg:DI 79)
(int_list:REG_BR_PROB 536870916 (nil)))
-> 11)
(insn 6 10 11 3 (set (reg/v:DI 74 [ b ])
(reg/v:DI 73 [ a ])) "pr105477.c":3:17 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg/v:DI 73 [ a ])
(nil)))
(code_label 11 6 12 4 2 (nil) [1 uses])
(insn 17 12 18 4 (set (reg/i:DI 10 a0)
(reg/v:DI 74 [ b ])) "pr105477.c":4:1 135 {*movdi_64bit}
(expr_list:REG_DEAD (reg/v:DI 74 [ b ])
(nil)))
(insn 18 17 0 4 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
The reload pass can then simplify as to the following:
(insn 2 5 3 2 (set (reg/v:DI 15 a5 [orig:73 a ] [73])
(reg:DI 10 a0 [77])) "pr105477.c":2:1 135 {*movdi_64bit}
(nil))
(insn 3 2 9 2 (set (reg/v:DI 10 a0 [orig:74 b ] [74])
(reg:DI 11 a1 [78])) "pr105477.c":2:1 135 {*movdi_64bit}
(nil))
(jump_insn 9 3 10 2 (set (pc)
(if_then_else (ne (reg:DI 12 a2 [79])
(const_int 0 [0]))
(label_ref 11)
(pc))) "pr105477.c":3:17 182 {*branchdi}
(int_list:REG_BR_PROB 536870916 (nil))
-> 11)
(insn 6 10 11 3 (set (reg/v:DI 10 a0 [orig:74 b ] [74])
(reg/v:DI 15 a5 [orig:73 a ] [73])) "pr105477.c":3:17 135
{*movdi_64bit}
(nil))
(code_label 11 6 12 4 2 (nil) [1 uses])
(insn 18 12 30 4 (use (reg/i:DI 10 a0)) "pr105477.c":4:1 -1
(nil))
The remaining two set statements above will become move instructions.
