WASM: Odd choice of selected instruction, and subsequent missed optimization #69938
Description
Instruction selection for WASM takes "advantage" of knowledge of byte alignment properties of the __stack_pointer global and prefers or instructions over add when it can. But the resulting code suffers from the fact that subsequent optimizations are unable to combine such or instructions with other additive operations, resulting in more instructions than necessary.
Example:
int foo(int *p);
int bar(double *p);
int f(int y[100], double a[100]) {
int x[100];
double z[100];
bar(z);
x[1] = 0;
int* xp = x + 2;
foo(xp);
return *(xp-1);
}$ clang -O2 -fexceptions -DAT -fno-unroll-loops -stdlib=libc++
f(int*, double*): # @f(int*, double*)
global.get __stack_pointer
i32.const 1200
i32.sub
local.tee 2
global.set __stack_pointer
local.get 2
call bar(double*)
drop
local.get 2
i32.const 0
i32.store 804
local.get 2
i32.const 800
i32.add
i32.const 8
i32.or <--------------------------------- or instead of add prevents folding
call foo(int*)
drop
local.get 2
i32.load 804
local.set 3
local.get 2
i32.const 1200
i32.add
global.set __stack_pointer
local.get 3
end_functionModifying the code a little creates a better code when compiler can't use or
int foo(int *p);
int bar(double *p);
int f(int y[100], double a[100]) {
int x[100];
double z[100];
bar(z);
x[1] = 0;
int* xp = x + 5; // modified
foo(xp);
return *(xp-1);
}$ clang -O2 -fexceptions -DAT -fno-unroll-loops -stdlib=libc++
f(int*, double*): # @f(int*, double*)
global.get __stack_pointer
i32.const 1200
i32.sub
local.tee 2
global.set __stack_pointer
local.get 2
call bar(double*)
drop
local.get 2
i32.const 0
i32.store 804
local.get 2
i32.const 820
i32.add <------------------------------------ add folded
call foo(int*)
drop
local.get 2
i32.load 816
local.set 3
local.get 2
i32.const 1200
i32.add
global.set __stack_pointer
local.get 3
end_function