0000157: Should use sub instead of add with large immediate

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Category

Severity

Reproducibility

Date Submitted

Last Update

0000157

[libFirm] x86 ia32 backend

feature

N/A

2015-09-09 08:34

2016-01-18 22:22

Reporter

Matze

View Status

public

Assigned To

Priority

normal

Resolution

open

Status

new

Product Version

Summary

0000157: Should use sub instead of add with large immediate

Description

For this example:
unsigned sub5(void) {
    unsigned x = rand();
    return x - 5;
}
We get the following code:
    call rand /* ia32_Call T[133:13] t.c:2:19 */
    addl $0xFFFFFFFB, %eax /* ia32_Add Iu[181:58] t.c:3:11 */
    ret /* ia32_Return X[143:23] t.c:3:2 */

This is correct but bad for code size: We really should not use an add with a 4 byte immediate but instead choose sub 5, xxx which is only a 1 byte immediate.

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Notes
(0000231) Matze (administrator) 2015-09-09 08:39	I just realized that immediate get sign extended, so this isn't really a code size issue but merely a cosmetic one.

Issue History
Date Modified	Username	Field	Change
2015-09-09 08:34	Matze	New Issue
2015-09-09 08:39	Matze	Note Added: 0000231
2016-01-18 22:22	Tron	Severity	minor => feature
2016-01-18 22:22	Tron	Reproducibility	always => N/A

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