https://llvm.org/bugs/show_bug.cgi?id=26291
Bug ID: 26291
Summary: MIPS back end performs inefficient subtraction of
immediates
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: MIPS
Assignee: unassignedb...@nondot.org
Reporter: csda...@swan.ac.uk
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Simple C test case:
int foo(char*,char*, char*);
int main(void)
{
char z[1048576];
foo(z,z,z);
}
The key part of the IR:
%z = alloca [1048576 x i8], align 1
This produces the following code for the stack pointer modification:
daddiu $5, $zero, 1
dsll $5, $5, 44
daddiu $5, $5, -1
dsll $5, $5, 20
daddiu $5, $5, -48
daddu $sp, $sp, $5
This would be much simpler as
lui $5, 16
dsubu $sp, $sp, $5
Even without rewriting the add as a subtraction, the constant could be
materialised as:
lui $5, 16
dsubu $5, $zero, $5
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